Also, to give you more accurate info on Elementalists and Mesmers Unknown_Gold...
Elementalist-focuses mainly on pure damage, especially along the Air and Fire lines. Air can cause a few status ailments, such as Blinding and Weakness, and Fire can cause Burning which degenerates health very quickly.
Earth and Water have more "support" skills, yet can indeed hold their own as far as offensive capabilites goes. They have the ability to have very high energy, yet can exaust it rather quickly, so energy management is key. A rather easier class to play, yet does take a degree of skill to manage your energy, and also deal with the AoE(Area of Effect) panic.
Mesmer-focuses on "shutting down" enemies, thus making it easier on the whole party. They also enjoy making enemies damage themselves by actions such as attacking or casting spells. Additionally, many skills are devoted to punishing foes for having energy, as well as even stealing their energy. They also have their share of HP dengeneration spells, and many skills that cause status ailments, such as Blinding, Deep Wound, and so on.
Much of their damage goes unnoticed, but if your group is taking down enemies quickly, likely your Mesmer directly contributed to that. A very fun class to play, but takes some thought and preparation to be effective.
So in summary,
E=Nuker, Warder, Spiker
Me=Shutdown, Energy Denial, Disruption
(search around if you're unsure of the build types)
I hope that helps clear things up a bit.
