I need some clarification regarding Big O
I have a few questions so please bear with me.  I need some help to clarify Big O and run time.  As I understand it Big O is a way of presenting the run time of an algorithm correct?  From reading I've been trying to figure out how to calculate the Big O of an algorithm.  So far I've figured out that something like this has a Big O of O(N^2)

    for(i = 0; i < N, i++)
        for(j = 0; j < N; j++)
          //code

But what happens if it is this:

    for(i = 0; i < N, i++)
        for(j = 0; j < M; j++)
          //code

Where N isnt always equal to M.

Also what is the Big O if you have two of these added together?

    for(i = 0; i < N, i++)
        for(j = 0; j < N; j++)
          //code
    for(i = 0; i < N, i++)
        for(j = 0; j < N; j++)
          //code

Is the big O equal to N^2 + N^2 = 2N^2?
        
I need some clarification regarding Big O
===

I have a few questions so please bear with me.  I need some help to clarify Big O and run time.  As I understand it Big O is a way of presenting the run time of an algorithm correct?  From reading I've been trying to figure out how to calculate the Big O of an algorithm.  So far I've figured out that something like this has a Big O of O(N^2)

    for(i = 0; i < N, i++)
        for(j = 0; j < N; j++)
          //code

But what happens if it is this:

    for(i = 0; i < N, i++)
        for(j = 0; j < M; j++)
          //code

Where N isnt always equal to M.

Also what is the Big O if you have two of these added together?

    for(i = 0; i < N, i++)
        for(j = 0; j < N; j++)
          //code
    for(i = 0; i < N, i++)
        for(j = 0; j < N; j++)
          //code

Is the big O equal to N^2 + N^2 = 2N^2?
        