Euler

Who is Leonhard Euler and what did he do? This is not Homework.
- Ali (age 11)
Toronto,Ontario,Canada

I suspect that your question was stimulated by today's beautiful Google logo in honor of Euler. For a reasonably complete answer, there must be articles on Wikipedia etc. Meanwhile, I'll mention just one beautiful equation due to Euler:
 

e^iπ = -1.
 

(published on 04/15/2013)