The main thing that separates death knights from other classes is that they operate using a completely different resource mechanic -- Runes and Runic Power. The three basic Runes -- Blood, Frost, and Unholy -- are a constantly regenerating or more appropriately, refreshing, resource that can be used to cast spells. Because they renew at a constant rate, the closest parallel resource to runes would probably be rogue Energy. On the other hand, Runic Power is generated when the death knight attacks or uses special abilities but decays over time through inactivity. In this way, one could imagine Runic Power being very similar to warrior Rage.
Death knights basically have two sets of spells that use two different resources. The advantage that death knights have over other classes is that Runic Power is the only one of the two resources visible to opponents, such that there's no way for players to know what Runes are off cooldown. Even if players were to memorize the Rune requirements of a spell, all three death knight trees have talents that can conditionally produce Death Runes which count as Blood, Frost, or Unholy Runes which complicates matters for those who like to keep track of these things.
Furthermore, death knights also have a few spells which require absolutely no resources such as Death Grip, a spell that very often sees use in PvP. Blood Tap is a spell that converts a Blood Rune into an active Death Rune and uses no resources, although demands 6% of the caster's base health. Then there's the Frost talent Lichborne, an extremely useful PvP talent, which doesn't use either Runes or Runic Power. As an added bonus, none of the aforementioned abilities trigger the global cooldown, which make them usable pretty much anytime.
It's very difficult to determine a death knight's full capabilities upon engagement solely on the basis of the available visible resource. As a general rule, death knights are at their most dangerous when they're loaded with Runic Power, although abilities have been rebalanced so as to no longer scale with it, such as Summon Gargoyle. A death knight with a lot of Runic Power can fire off two or even three Death Coils in quick succession, or use other abilities also popular in PvP, such as Icebound Fortitude and Mind Freeze. Because Runic Power decays, death knights often want to use abilities that use up Runic Power, so be wary. That said, even a death knight with 0 Runic Power can build it up relatively quickly.
A plateful of diseases
Another interesting feature of death knights is that they deal a mix of physical and magical damage, similar to retribution paladins. They also make use of a previously uncommon debuff type, Diseases, which plays a huge part in death knight damage -- or at least the unholy spec, which is popular in PvP. Classes able to remove diseases such as
druids paladins (sorry, missed that one! thanks, folks!) and shamans have fair defense against them, and it's often a good strategy to spend some mana and a few global cooldowns removing their debuffs, which can prove fatal if allowed to stack and run their course.
Death knights are a plate-wearing class, which allows them to withstand a lot more physical damage. On top of that, they have a few healing abilities that allows them to heal themselves or their undead allies. Don't be surprised to have a death knight be low on life and suddenly be rejuvenated. Regardless of spec, a death knight can use Death Pact to sacrifice a minion such as a ghoul or gargoyle to regain 40% of the death knight's health, as well as use Death Strike to regain health while dealing damage. blood-specced opponents, in particular, can generate a lot of health using various tricks. We'll get to that when we take a look at each spec a little more closely.
Although most effective at close range, death knights have a few means by which to close the distance, most notably the fearsome Death Grip. A lot of less experienced death knight players will use this as an opener, which can work to your advantage as long as you are mindful of the ability's long cooldown of 35 seconds (or 25 with Unholy Command). Another important snare ability is Chains of Ice, which uses up a valuable Frost Rune. Dispelling the first Chains of Ice is a good way to waste a death knight's Frost Rune. It's a Magic effect, so classes that can dispel magic such as priests or paladins should do so instead of using a trinket or Hand of Freedom. Frost Runes are needed for some important strikes such as the unholy talent Scourge Strike or Obliterate, so many death knights will be judicious in their use of Chains of Ice, although they won't hesitate to use it often to keep enemies in place.