Answered elsewhere I think. But basically there are open source fonts now (Roboto, Open Sans, Liberation, Adobe's Source family) as well as distinctive fonts like Ubuntu, and a legacy way to get the core fonts.
Linux can (with quite a bit of pain) use font alternatives to swap in Liberation Sans for Arial or whatever (at least I assume that is what is happening)
Linux can (with quite a bit of pain) use font alternatives to swap in Liberation Sans for Arial or whatever (at least I assume that is what is happening)