Ok my quick overview of what I think the classes are good at cos it's probably a bit confusing and i've had a bit of a mess around no most of them, and played vs all of them. Bearing in mind, all classes can be tweaked to play whichever role you prefer to play (survivability/tanky, Direct damage dealer/damage-over-time dd/ burst dmg nuker, area-of-effect damage/support/ crowd controller = knockbacks,stuns, controlling enemy movement basicly.) and you can literally customize every class in SO many ways based on weapons (if you didn't know, your 5 attack skills are based on the weapons your holding. 3 for mainhand 2 for offhand and 5 for 2handed), runes you put in them later on, and obviously your 'utility' skills which are a heal, 3 utility, and an ultimate.
Mesmer - tricksy range dps with some good burst and sustained damage through using clones. easy to spot real mesmer, clones are just for extra damage. don't rely on them to confuse someone, but if the clone doesn't die you can do a lot of damage as it will use the ability again if it doesnt die unlike every other class where the skill is on cooldown. mainly a direct damage.
Thief - tricksy melee dps with a lot of gap closers, but the most common build is a dual pistol build, but melee s still good. can play a very supportive role, can crown-control enemies well, or be a squishy but reliable dd. mainly a direct damage class. very versatile though with good weapons options
Guardian - massive variety of playstyles but not as good at dealing damage as other classes. can be built very tanky, very supporting, or a mixture also having crowd control. mobility can be a problem but can be overcome. Mainly a supprot style class.
Ranger - excels at buffing himself and teammates (speed buffs are almost necessary in the Mists, pve not so much). very good melee builds and melee/ranged damage-over-time based builds. Can make a mixture of both dot/melee buidls and stack an isnane amount of dots almost instantly. its enough to '1shot' people witha single combo. 1 trick poney, completely OP and will be getting nerfed, but the build will still exist. Good ranged and melee damage dealer with great support.
Warrior - king of all weapons. most versatile class. can be a supportbased healer, a ranged dot damage dealer, or any type of melee; mobile burst, less mobile great stuns/crowncontroller, lots of sustained dmage and it's profession mechanic is a massive burst damage nuke ability based on weapon type. Has a certain combo that involves immobilizing/stunning someone, charging them with a utility skill and literallty 1'shotting someone which isn't meant to happen in MMO's, even GW2. it's not a 1 trick poney build either, and it's not OP if you know hwo to counter it. there will be a lot of warriors at launch and fr good reason, they are very strong melee dmage dealers with the best burst, but can also be played in almost any way you want.
Necro - mainly a supportive class with a lot of condition damage (over time, putting negative effects on enemy) can still pack a punch and is possibly the most survivable of all classes having basicly 2 health bars once 1 is down. You do NOT want to chase a low health necro that knows what theyre doing
Elementalist - ranged nuker/damage dealer that can also play the typical 'healer/support' role very well. has 4 elements, can be in 1 ata time but basicly has a dmage dealing, heal/support, a damage/support and a d-o-t/crowd control mode to choose from. versatile but not as much as warrior. can be a very strong melee. can have great ranged aoe damage/support abilities.
Engi - good ranged damage dealer with great crowd cntrol, probably the best. sets up turrets to control movement and/or support teammates with heals etc. Very useful class as its one of the few that has a 'pull' that can affect multiple targets.
Builds for every class are based on what weapons you want to use (what skills you want on your main buttons) how you stat your trait points (like levelling in a normal mmo) and which utility abilities you choose (max of 3, with 20-30 or so to choose from. very defining of your playstyle and your class).
You can literally take any class and make it how you want. there is no 'this is a ranged class' 'this is a support' etc.
Hope this helps. Yes, I'm bored and need this game in my life. No, I have not been outside today