Guild Wars 2 looks pretty great, but I'm still not sure how combat functions. Is it more like TERA, where you have to aim and dodge, or is it more like WOW, where you just select a target and spells are guaranteed to hit (assuming 100% hit)? Does combat function the same for ranged and melee combat?
I've not played TERA, so I can't do a comparison. I've also only played BWE1 of GW2 recently and I tried Guardian and Elementalist.
Aim and Dodge
Guild Wars 2 has been specifically designed with active combat in mind. As such you must keep moving at all times when in combat, either by circling your enemy, backing up, or using Endurance to dodge.
If you stand toe-to-toe with enemies you'll get put down quickly.
If you can circle enemies and stay mobile it's possible to put down +3's without even taking a hit! I played a Guardian in the BWE1 and using a Scepter & Shield combo it was possible to take on large groups without taking much damage just by making sure to run around lots.
The main thing to remember is the 5 D's of Guild Wars 2, dodge, duck, dip, dive and dodge! Just remember that your Endurance is limited and you'll only be able to dodge about 3 times in a row before it'll run out. I would strongly suggest strafing around enemies, even in melee and only using dodge to avoid large attacks.
As the developers have said, the combat is designed to be very active and you'll need to watch your enemies animations to spot when attacks are coming in order to evade them. The best example is Ettins using their big smash attack, where the animation starts with a gathering flash around them before they launch the attack. It's visual cues like this which are your keys to using dodge effectively. Check out the Ranger's Serpent Strike skill video.
Also be sure to understand the ground markers. White are positive effects and will usually grant a boon (positive buffs) to you. Red markers are enemy AoE markers and will usually damage you of give you conditions (negative debuff), so you will need to run out of these areas or use dodge to roll out of them.
Targeting and Hitting
As for aiming, your attacks are always targeted at your currently selected target. So as long as you have an active target you will not need to target your attacks. There are a few types of attacks which do need targeting, usually Targeted AoE attacks. When these skills are activated they will change your cursor into an image of the area, which will allow you to place the skills effect. Do be aware that there is a fast-cast option in the game options which will activate these skills at your current cursor position, thus avoiding the need to hit the skill and then place the effect. These types of skills also apply to walls, which in some cases are automatic, like the Guardian's Line of Warding and some are placed like the Elementalist's Flamewall.
Do be aware that if you do not have a selected target you will still be able to activate skills and they can damage enemies but you'll struggle to deal proper damage and in most cases will die. This happened to me a few times in the BWE1.
Ranged and Melee
This is a hotly contented issue and spawned a huge thread on the forums. Mainly as most people felt that Ranged was far more useful than Melee in the BWE1. The developers agreed with this and said that the dynamics of ranged vs melee will be tweaked as the game progresses.
The function of the two modes of damage dealing are the same. They both have a range, which you can see by looking at the skill bar, if the number is white, you can use the skill, and if it's red you are out of range.
There are a few points to note with some things such as scepters shoot a projectile which tracks the ground, thus you are unable to fire it up or down. Rangers have a huge range and pretty much everyone just stuck with Long Bow in BWE1. I ended up using Scepter on my beta Guardian simply because it was easier than using a melee weapon. I tried Greatsword, Mace, Hammer and Sword, but none of these were as easy. Don't get me wrong, they were all effective, but you'll take more hits from melee enemies as you can strafe/kite them around.
If you read all this and got here, reward yourself with a brew and a bikkit.
I'll add a few things to what Raven Dreamer said
First, skills in Guild Wars 2 are definitely not 100% hit. You do have to aim and dodge, like in TERA. There are no enemy-targeted skills in Guild Wars 2, you aim "in the direction of" you enemy. If you are out of range, if your enemy moves, or if something else moves in between, you will miss that enemy. So it's entirely possible for someone else to jump between you and your target, and take the hit to save his fellow player. Or someone can throw up a defensive wall which will stop your projectiles. Or the player could dodge behind a rock. That is an essential part of the dynamic combat in Guild Wars 2.
Second, it's more than just the lack of a dedicated healer class. Although every class has support abilities (including some healing), none of the healing skills are powerful enough to be able to play a dedicated healer. The most powerful healing skills are self-healing, so taking responsibility for your own health is crucial. Contrary to the standard Holy Trinity model, it's also impossible to play a dedicated tank. There is no way to hold aggro for long, and trying to play a tank and have an enemy focus entirely on you will undoubtably get you killed very quickly... especially at higher levels in the game. Your current role is determined by the (skills on the) weapon that you wield, and you can dynamically switch roles (support, damage or control) by switching weapons. Learning to do this at the right times is essential in order to master combat and survive at higher levels in the game. But at lower levels combat is fairly easy, so it does give you plenty of time to get into this and learn it.
All this means that combat in Guild Wars 2 is a lot less intuitive than it is in TERA (at lower levels people will easily have the habit of treating it like WoW combat and trying to stick to specific roles), but it also means that you have a lot more options and flexibility. It takes a while to really learn this combat system, but once you get the hang of it, it's an awful lot of fun.