New Forum

Not particularly sure if this is the right section to post such a question. But I am seeking advice regarding the choice of a new forum for my class. First let me make this clear : I know nothing about how forums are developed and run.(-:
I heard there are many readymade forums like Invision and PHPBB. The thing is I need one that should be 100% free. Is the Invisionfree forum that they offer completely free ? What would be its limitations ? Are there other better options ? Can anybody shed some light into this…?