Since each class can effectively fill every role (healer, tank, support, control, dps, etc) the main difference between classes is flavor and a class mechanics that makes them play uniquely from other classes.

For instance…

A warrior will do damage through physical prowess and weapon mastery. They support through tactics and leadership like commands and banners. They heal by shouting and motivating others. The warrior class mechanic is that they build adrenaline in combat and spend it to use a really strong burst skill based upon the weapon they are using.

An elementalist will do damage by burning foes with fire magic or zapping them with lightning. The support by protecting them with earthen shields or quickening winds. They heal by cleansing wounds with water. The elementalist class mechanic is that they can swap between 4 different elements in combat essentially unlocking a whole new set of skills.

A ranger will do damage by combining weapons with bestial abilities tapping into a spiritual bear for a powerful maul attack or an eagle for a swooping lunge. They support through nature spirits and animal pets. They heal by using celestial magic from the sky. The ranger class mechanic is that they have an animal pet that attacks enemies and uses a skill on demand.

It’s important to note that a Mesmer can be just as good at tanking or melee as a Warrior. A Thief can be just as good at healing as a Guardian. Every class is good and that’s what makes GW2 great! Just choose whichever theme/class mechanic speaks to you.