There are a lot of sites that you can access to find out this information. Try www.studentdoctor.net and on the first page they have a link that describes briefly what a DO is.
They are equivalent to MD's in the U.S., though they have separate schools. They learn all of the same medicine that MD's do and, additionally, learn something called OMM (osteopathic manipulative medicine). It's using the hands to help encourage healing (sort of like chiropractic, but not exactly). They are in all of the same specialties as MD's, but tend to be in primary care.
It's much more difficult to gain entry into a DO school compared with a DPM school. MD schools are by far the hardest, though.