Q:

what is the difference between O(nk) and O(n+k) in time complexity?

In big O notation of time complexity in algorithmic analysis, when an algorithm depends on n and k, what is the difference between these two notations. 
Also pls help in the notation to use if there is a nested loop with outer loop running n times and inner loop running k times ?

A:

O(nk):
for( i=0; i<n; i++ ) {
   for( j=0; j<k; j++ )
   {}
}

O(n+k):
for( i=0; i<n; i++ )
{}

for( j=0; j<k; j++ )
{}

