What are public land titles?

I have been reading about free land offers and public land. what do you need to do to qualify like do you have to be a farmer or a cattle rancher or can it just be anyone just looking for land? what are the restrictions living on govt land? and is the only expense $20 a year for land tax? cause that seems way too good to be true. if you could include where you got your info that would be great. thanks :)