Euler's Number??

Question

What is it? And where did it come from?
(HELPFUL websites are welcome! :-))

Answer 1

1+1/2+1/3+.....+1/n = c +ln(n) +a_n where lim a_n = 0 and c is Euler´s consttant

Answer 2

http://en.wikipedia.org/wiki/Euler_numbers