The first thing you need to know is how to make and use a deck. To put it most simply, you need to collect the ace card and the number cards 2-8 of a single card type, "use" one of them to combine the cards into a single deck, take the quest from the deck, and turn the it in to Professor Thaddeus Paleo, who hangs out at one of the booths. WoWWiki has an excellent, concise page on which trinkets come from which decks, and where to find all the cards. Be sure you know whether you're after a level 60 or a level 70 deck, as both have slightly different rules as to which mobs drop their cards. The level 60 number cards only drop from level 50+ humanoid mobs on Azeroth, and the aces drop off specific bosses in specific Azerothian dungeons. The level 70 number cards aren't quite as limited in range, and drop off almost any level 70ish mob in Outland, while the aces drop off almost any level 70 5-man dungeon boss.
But with 8 of them, it can certainly be a bit overwhelming deciding which card, if any, is right for you. Each deck definitely has its pros and cons, and while some of the 60 trinkets aren't quite as useful anymore since the release of the Burning Crusade, there's others that might still be worth the trouble. We've said a little bit about the cards before, but now it's time to get down to the nitty-gritty. We'll discuss those level 60 trinkets first, then proceed to look at the level 70 cards that came on the heels of the expansion.
Level 60 Trinkets
Trinket Name: Darkmoon Card: Blue Dragon
Description: Equip: 2% chance on successful spellcast to allow 100% of your Mana regeneration to continue while casting for 15 sec.
Who can best use it: Caster classes with lots of spirit that use short-cast and instant-cast spells.
Pros: This is probably the level 60 card that holds up best at level 70 since it works off a percentage of your mana regeneration, and thus scales no matter how much you have. It also makes a great pair with the Bangle of Endless Blessings, giving you more chances to have some sort of extra mana regeneration at any given time. It's especially amazing for Tree Form Druids, since they usually have a lot of spirit from gear and talents anyway, and cast spells with a short or non-existent cast time.
Cons: Since it will primarily work by bypassing the "5-second rule" that prevents you from regenerating mana from spirit while casting and just after casting a spell, It's not really worth using if you don't have quite a bit of spirit. It may also be a pain to get a group together for The Beast to get the Ace of Beasts you'll need to make the deck since the UBRS Dungeon requires a rather hard to get key. If you're fighting a lot of short fights where you don't need much mana regeneration, you may want to equip a trinket that'll give you damage and healing or some other benefit that doesn't depend on a proc.
Trinket Name: Darkmoon Card: Twisting Nether
Description: Equip: Gives the wearer a 10% chance of being able to resurrect with 20% health and mana.
Who can best use it: Anyone, although it's traditionally been used as an "extra soulstone" by healers.
Pros: It's always nice to have a second chance, and that 20% figure will scale no matter how much health and mana you manage to get. In PvP, it may allow you to get the jump on an opponent who thinks they've won, and even in grinding, it may at least save you a long corpse run. In grouping and raiding, especially on a boss that your group doesn't quite have on farm status, it could save you a wipe, or give you a chance to resurrect and start bringing people back if the Warlock forgot to use his Soulstone.
Cons: It's only a 10% chance, so there's a lot of deaths where it just won't work. You'll have to decide whether the chance to come back is worth losing whatever benefits other trinkets would give you.
Trinket Name: Darkmoon Card: Heroism
Description: Equip: Sometimes heals bearer of 120 to 180 damage when damaging an enemy in melee.
Who can best use it: Any melee class.
Pros: It's nice to have a bit of passive healing while you're grinding or farming primal motes, and it doesn't seem that the rate or power of the healing of this card has been lessened for Burning Crusade in the same way the Crusader enchant has. There's a bit of an upgrade for this in the Mark of Conquest, which is a PvP reward out of Zangarmarsh for either side, or you can combine both for faster regeneration. It also works for Feral Druids, and can proc off multiple Swipe hits.
Cons: The heals are a bit low powered for a group or raid setting, especially with the astronomical climb of stamina and health totals since this card was first implemented. It also doesn't seem to work off ranged hits, so hunters probably should stay clear of it, unless they're disciples of Gweryc.
Trinket Name: Darkmoon Card: Maelstrom
Description: Equip: Chance to strike your melee target with lightning for 200 to 300 Nature damage.
Who can best use it: Any melee class.
Pros: Since it's counted as nature damage, it can do extra damage on nature-vulnerable mobs, and like the Darkmoon Card: Heroism before it, works for Feral Druids and can proc off multiple Swipe hits.
Cons: It's probably aged the worst of the old Darkmoon Cards, and you may find that an attack power or critical strike rating trinket will give you more of an overall DPS boost. Also, the Maelstrom card doesn't work for ranged attacks, and since it's counted as nature damage, it will also be ineffective against nature immune or nature resistant opponents. Romulo's Poison Vial, off the Romulo and Julianne Opera event in Karazhan, is a clear upgrade for this trinket if you really like having that nature damage proc.
Level 70 Trinkets
Trinket Name: Darkmoon Card: Vengeance
Description: +51 Stamina, Equip: You have a 10% chance when hit by an attack or harmful spell to deal 95 to 115 holy damage to your attacker.
Who can best use it: Tanks, especially Paladins.
Pros: With a ton of stamina and a "chance when hit" proc, It's a good card for any tank. However, it's almost a must have for a serious Paladin tank, since the holy damage will cause massive amounts of extra threat via Righteous Fury. In addition, Paladin tanks often find themselves taking on large amounts of mobs at once, so they'll have more chances to activate that damage when hit as well.
Cons: if you're not getting hit much, you won't be getting most of the benefit of this trinket.
Trinket Name: Darkmoon Card: Crusade
Description: Equip: Each time you deal melee or ranged damage to an opponent, you gain 6 attack power for the next 10 sec., stacking up to 20 times. Each time you land a harmful spell on an opponent, you gain 8 spell damage for the next 10 sec., stacking up to 10 times.
Who can best use it: Any DPSer, especially those with a fast attack speed.
Pros: This trinket is definitely amazing, since if you can keep the buff constantly refreshed, you'll have 80 extra spell damage, 120 extra attack power, or both. If you do PvE DPS and rely primarily on high attack power or spell damage to do your thing, you should strongly consider getting this trinket. On any battle that lasts long enough for you to work up to a full stack of the effect, you'll be doing a very nice bit of extra damage. Enhancement Shamans and Retribution Paladins can also rest assured that the effects stack with each other, so you'll be able to hit harder with your weapons while also getting a bit of extra oomph for your Earth Shock or Judgment of Command.
Cons: You need to make sure you're hitting a target at least once every 10 seconds to keep the buff refreshed, and misses, resists, and immune attacks don't count towards activating or refreshing the buff. This means it can be a bit of a pain to keep the stack refreshed on bosses that go immune at any time, such as Moroes, or on fights where you have to run around a lot, such as, say, arena battles. If you use a slow weapon or use spells with long cast times, it may take a while for you to build up to a full stack.
Trinket Name: Darkmoon Card: Wrath
Description: Equip: Each time one of your direct damage attacks does not critically strike, you gain 17 critical strike rating and 17 spell critical strike rating for the next 10 sec. This effect is consumed when you deal a critical strike.
Who can best use it: Any DPSer who relies on critical strikes for damage.
Pros: If you're a DPSer who relies on critical strikes to get their damage in, and haven't been able to get a good critical strike percentage, this could be your trinket. It might also allow you to swap out some critical strike rating on your gear and gems for a bit more spell damage or attack power while not sacrificing as much in the way of total critical strike percentage. This can also be useful in PvP for giving you an edge against opponents with a high resilience rating.
Cons: Once you start getting a high critical strike rating, this trinket will be less useful, since you'll be constantly critically striking anyway. It's another stacking buff, so like the Darkmoon Card: Crusade before it, you'll have to be making a successful direct damage attack every 10 seconds for the stack to stay up.
Trinket Name: Darkmoon Card: Madness
Description: 51 Stamina, Equip: Each time you land a killing blow on an enemy that yields experience or honor, you gain the Power of Madness.
Who can best use it: Everyone!
Pros: The "Power of Madness" buff is actually a collection of various buffs that include haste rating, attack power, some extra stamina, and more. These buffs are also limited by class, so you don't have to worry about getting the +spell damage buff if you're a Warrior. Also, the stamina means that even if you don't get the buff, you'll be getting a good benefit from the card anyway. However, the rest of these benefits pale compared to the the best part, the fact that you will randomly say, "This is madness!" when you trigger one of the buffs. Not only that, but the other faction can hear you say it too. Of course, that means that you have to hope that the big hulking Tauren Warrior next to you doesn't respond by kicking you into a nearby bottomless pit.
Cons: The effect is random, so you can't count on a certain effect when your target dies. The buff is also only 30 seconds long, whatever it is, so if you're not killing a lot of targets in a short amount of time, it could be lost. You also have to make sure you're getting killing blows to activate the buff, which makes it harder to use for groups if you're not a burst damage class.