Each build has a number of factors that affect it:

• Gear stats. Different gear sets have different stat sets on them, though the PvE meta heavily focuses on one or two of the gear sets.
• Weapon selection. Your weapons determine most of the skills you'll be using that deal damage, so you'll want to choose the right ones to synergize with your build. Do note that you can swap between 2 weapon sets in combat on most classes (though not all). Every class has a different selection of weapons they can use, and every class uses those weapons different (for instance, while a warrior uses a greatsword as a melee weapon, a mesmer uses it as a ranged weapon).
• Utility skills--the 6-0 skills--can be picked based on what type of build you're going for. They have a fairly good variety of skills, and you can pick and choose between them at will. These include your heal skill, 3 utility skills, and your elite skill. Each class has completely unique utility skills.
• Sigils are slotted into weapons (2 sigils per weapon "set"), and provide bonus effects whenever you swap weapons, land a crit, hit an enemy, get a kill, or one of a few other triggers.
• Runes are slotted into each piece of your armor, and give you a bonus to a few stats, can affect conditions, and can give a few of your skills (like your heal or elite skill) extra effects.
• Foods (varying consumables) can provide effects varying from granting stats to granting bonus effects on crit.
• Traits provide huge effects that can completely change the way you play. They can boost certain weapons, boost certain types of utility skills, provide additional effects on hit or on crit, interact with each class's unique class mechanic, and provide any number of effect. There are a total of 80 traits available to each class, divided between five unique "lines" per class. Adding trait points to each line also gives you some bonus stats.