Freeform basically let's you play with any deck you like to build without any restricted or banned list in place (even without the 4 of a kind maximum restriction).
However, a lot of players use the freeform room for the Pennydreadful format or for the Premodern format. Both non-official formats with a predefined b&r list and allowed sets.
Information about Premodern (including banned list and legal sets) can be found at:
No, you can play a deck with any card that you got in your collection. Play in the freeform room (as in all open, non-tournament rooms) is free, no entry free charged, but you still need to have the cards in your collection.
