DPS
These specializations focus on personal damage output. They will have both good single-target, and area-of-effect damage located in the tree. They generally are lacking in defensive skills and crowd control abilities. Some ranged dps will sacrifice damage for mobility. Some melee dps will sacrifice mobility for more defenses.
Offensive Supports
These specializations focus on increase the damage of their entire party. They have higher damage than other non-dps specializations. Their toolkits often involve improving the offensive stats of your groupmates, reducing the defenses of your enemies, or providing on-hit effects. Melee offensive supports have more self defenses.
Tanks
These specializations focus on redirecting enemy damage towards themselves. They deal average damage and usually are lacking in area of effect damage. They can have effects that are suppressed whenever they are hit, they have abilities such as guard, usually potent defensive skills. Some tanks have some support functions, while others have high amounts of crowd control.
Defensive Supports
These specializations focus on reducing the damage their allies take. They deal the least amount of damage, and they have high defenses. Some defensive supports increase the defensive stats of their allies, or reducing the offensive stats of your enemies. Some defensive supports have a high amount of crowd control. Some defensive supports utilize many absorb effects. Defensive supports can have some minor health restoration ability.
Healers
These specializations focus on restoring health. These skills can be standard healing skills, or lifetaps; but the majority of their toolkit will restore health. Melee healers have effects that allow them to survive splash/cleave damage in combat without the need of guard; but at the cost of missing ranged healing output.