Most free email providers make their money by showing you advertisements, either graphical banners that you see as you collect your mail from their site, small text messages that appear at the end of your email messages or even advertiser messages that are sent directly to your emailbox.
Some smaller providers offer free email as a community service, without any advertising whatsoever.
Finally, some large sites offer free email as an attempt to lure visitors back to the site again and again. A few email services get a bit carried away sending you lots of advertisements. Also, you need to remember that the Web changes very quickly, and some providers disappear from one day to the next. If possible, give your most important contacts a second way to get in touch so that they will never lose contact with you. You'd probably get a bit confused, though.