First of all, the free in free software means freedom. Users will have freedom of getting and editing software source. Opensource software is a kind of free software. Because Opensource means showing source code to everyone. But that doesn’t mean you could use every Opensource software without cost. Opensource licenses define which situation is with payment or not.

So in one sentence, Opensource software is a kind of Free(freedom) software, but is not alway free(without cost).