You'll hear about him when you study calculus. The constant e bears his name. Euler's identity, or the formula
eiπ + 1 = 0
is called the most famous formula of mathematics. It ties together the important numbers 0, 1, e, i, and Pi.
Euler also discovered the formula V − E + F = 2 relating the number of vertices, edges, and faces of a convex polyhedron.
A lot of the notation we use is attributed to him: for example, Euler was the first to write f(x) to denote the function f applied to the argument x. He also introduced the notation i for the imaginary unit and the Greek letter Σ (sigma) for summation.
Then, if you study graph theory, you'll immediately encounter the famous problem about the Seven Bridges of Königsberg. Euler solved that, as well.
He also proved many results in number theory. And on and on and on... he wrote volumes and volumes of works in mathematics, and is a fellow worth knowing something about, I feel!