where to use which notation

Hello to all........

I'm new to this subject, and currently i'm learning asymptotic notations. The problem is that i'm confused about the use of these notations. I mean where to use which notation.

For exp. the complexity of insertion sort is O(n^2), why not o(n^2) or omega(n^2).

Plz understand my confusion and explain it.

Thanks in advance!!!!!!!!!!!!!!!!!!!!!!!