Mesmers are of the class Scholar. Just like the other scholars, they are focused on studying some part of the world. Elemetalist studies the elements (duh right?) and Necromancers studies the dead.
But what does Mesmer studies?
Mesmers are scholars studying people. People is not limited to humans. According to the Disney’s Pocahontas, “you think the only people who are people are the people who looks and think like you.” Mesmer obviously shares the same belief that people comes in different shape and form. This means that Mesmers studies all different kind of people in Tyria; from Humans, Charr, Sylvari, Skritts, Hyleks, etc., even the undeads of Orr.
So what do they get from studying the different people?
If you understand the people’s behavior, you get to control their minds. You can insert ideas that manipulates their abilities by either positive reinforcements to make people do better or take advantage of their fears to make them fail. What Mesmers found in their studies is that although people in Tyria are different, they all share a common fear — that is being outnumbered. Mesmers takes advantage of this fear by forming their magic to create illusions of themselves in battle.
While Elementalist and Necromancers can summon ugly minions, Mesmers only surrounds themselves with pretty people.