Question

what is (fog)(2) ? Whats the difference with fog?

Answer 1

The "o" is mostly writte smaller, to make it not look like a letter.
It is an operator, it means: "after", so (f o g)(2) means: calculate g(2), then put that in f:
so f(g(2)).
f o g on itself means th same, only it doesn't require you to actually calculate a value.