Most fonts are copyrighted; different licences apply. Normally the font comes with an agreement that tells you what uses are and aren't okay. Some are quite restrictive and involve large fees to use, some are completely free for any purpose - most of the best fonts do have a fee attached for commercial use, but it is usually a moderate, reasonable fee (often around USD $30).
Generally though, the thing is to check any documentation that came with the font(s), and if in doubt a google search for the font name may well come up with the terms.
If you particularly need free fonts, then Googling for the relevant terms will get you a lot of good results. You'll still need to read the terms as free to download doesn't always mean free for commercial use.