There are variations to those builds and by no means are they the end-all-be-all of specs, but if you're not sure where to start they're good baselines. I'm also not covering the "tanking shaman" build here, which personally I think is an awesome side-bar but not something we're going over today.
I'll run through all of the enhancement tree and a select few of the elemental tree to review the specs. I'll mark essential talents with [PvE] or [PvP] if they are absolutely essential for a PvE or PvP build, respectively.
Enhancing Totems - 3 points - [PvE] - A death knight can offer the raid Horn of Winter which matches a Strength of Earth totem without the talent points, but our improved totems trump their and is a must have for PvE. Buff yourself and your mates with 3/3 in this for PvE. As for PvP it's usually skipped because you're not usually using your Strength of Earth and Flametongue totems.
Earth's Grasp - 2 points - [PvP] - Absolute must-have for PvP for the two benefits: It boosts the range on the Earthbind Totem and more importantly it reduces the cooldown on Stoneclaw Totem, which we'll get into when we talk about glyphs next week. This could potentially be useful for leveling, though really your points are better invested elsewhere and it has no place in a raid environment.
Ancestral Knowledge - 5 points - This is the filler talent of tier 1. I didn't list is as required, but in truth we use it to fill out and get down to tier 2. Put in 2/5 for PvE builds and 3/5 for PvP builds. Maxing it out to 5/5 is not recommended.
Guardian Totems - 2 points - This is a very much a skippable talent for most PvE and PvP. There are instances where it could be beneficial, but for the most part I'd discourage any enhancement shaman from investing in this; we have a lot of ground to cover on other talents below.
Thundering Strikes - 5 points - [PvE] [PvP] - Core talent. Without this you're just not an enhancement shaman. And you can't get down to flurry. No question about it, 5/5 here.
Improved Ghost Wolf - 2 points - [PvP] - Insta-wolf! No better way to hunt down opponents, or escape them in PvP than insta-wolf. And since Ghost Wolf form was buffed to be "less hindered" by movement slowing effects the ability to pop into it immediately is crucial.
Improved Shields - 3 points - [PvE] - I did list this as an essential PvE talent, but a full 3 points are not required. Use this as a filler for your talents. If you find you're never out of mana then you pull some talents out of some of the mana conservation talents and put a few more in here. Even the most mana conservative build should be able to find room for 1/3 IS. Keep in mind that every point spent here is approximately an additional 100+ dps.
Elemental Weapons - 3 points - [PvE] [PvP] - Core Talent. 3/3 here for additional damage
Shamanistic Focus - 1 point - This plus Improved Stormstrike and Elemental Focus (both covered below) are the mana conservation/regeneration talents we can mix and match -- not counting Shammy Rage or Mental Quickness which are a core talents. Investing fully in all three would require 4 talent points, but many enhancers including yours truly can get by with only 2 points across these talents. So I list this as optional depending on where you want to spend your mana talent points. Personally, I skip SF on Stoney.
Anticipation - 3 points - This is not the best choice for PvP talents, though it is certainly an option. First, disarm is not often a problem in PvP. I've found I'm usually treated as a kill-on-sight target in PvP -- just by nature of being a shaman -- than considered a threat, so I don't often find myself disarmed. Thus, mitigating something that doesn't happen often isn't a great use of talents. Second, the dodge becomes a liability against warriors who can Overpower after a dodge. I usually skip this entirely.
Flurry - 5 points - [PvE] [PvP] - Core talent. It's one of the things that makes enhancement, uh, enhanced.
Toughness - 5 points - [PvP] - I list this as a core PvP talent, but rarely do I see experienced PvPers invest 5/5 in it. If you're low on PvP gear or find yourself dying too quickly then you may want to look into investing more, but most of the top rated arena enhancers go 2/5 or 3/5 into Toughness. Use it as a filler for PvP, but definitely use it.
Improved Windfury Totem - 2 points [PvE] - I list this as a core PvE talent even though it is equal to a frost death knight's Improved Icy Talons. If you do not have a frost death knight in your raid or party then all of the melee -- including you -- is missing out on an extra 4% melee haste. I've seen a few hard-core 25s-only builds that invest the 2 points elsewhere and if you are only running 25s and will always have a frost dk. For the vast majority of us that run 10s and 25s and can't always count on a frost death knight, invest 2 points here.
Spirit Weapons - 1 point - [PvE] [PvP] - Core talent. There was a time when this only reduced threat on melee attacks and an enhancer's primary threat reduction was a little spell called Reincarnation. Thankfully this was expanded to reduce threat on all damage dealt.
Mental Dexterity - 3 points - [PvE] [PvP] - Core talent. MD is what makes mail gear usually superior to leather. Take 3/3 now and wonder how it will change come Cataclysm.
Unleashed Rage - 3 points - [PvE] [PvP] - The AP benefit is equal to a dk's Abomination's Might or a hunter's Trueshot Aura but besides that this is an essential talent for the 9 points of expertise we get from it. In the event there is not a death knight or hunter with those abilities then UR will pick up the slack.
Weapon Mastery - 3 points - [PvE] [PvP] - Core talent. Extra weapon damage? Yes please.
Frozen Power - 2 points - [PvP] - This talent was brought to us specifically for PvP usage. Please note that it is ineffective against PvE bosses so don't bother with it in builds outside of PvP -- and if you see an enhancement shaman in PvE with 2/2 FP please point them to this article. For PvP definitely drop 2/2 in here and root those foes trying to flee you. To me there's a sweet satisfaction to rooting a frost mage in ice. Sweet justice! Even if only for a moment.
Dual Wield - 1 point - [PvE] [PvP] - Core talent. We're dual wielders. It's kinda what we do.
Dual Wield Specialization - 3 points - [PvE] - Absolutely essential for PvE. No question about it. With out this talent you'll just be gemming for hit and frankly you can't make up the lost gems with extra dps from talents invested elsewhere. But why not a core talent for PvP? The percent miss against a target of equal level is significantly lower than that of a boss so some experienced and top rated enhance shamans only go 1/3 or 2/3 into DW Specialization. This is a judgment call, go with what your most comfortable here though I'd recommend 1/3 at a minimum.
Stormstrike - 1 point - [PvE] [PvP] - Core talent. Love the Stormstrike.