C.,
I have taken a few classes from UOP online that I needed to transfer to a program at my "land-based" college. UOP is an accredited institution and that makes it legit. I had moved away from my university and needed to take a few online to transfer to get my Master's degree so I didn't have to drive a long distance. There are many online colleges that are not accredited so you do have to be careful, but Phoenix is accredited.
This link may help: http://www.back2college.com/library/accreditfaq.htm
All colleges that are accredited in the United States are doen so by one of six regional accreditation agencies. A college's website should tell who accredits them. It's not to say that any class or program from a non-accreditated place isn't helpful; however, if you are pursuing a degree for a certain career or taking a class for transfer, many employers or other colleges require it be from an accredited place.
Also, I have found that my online classes are just as much work (if not more!) than a traditional face-to-face classroom format because it requires a lot of self-discipline to devote the necessary time. Some people have a hard time not having the personal interaction, but most classes have discussion boards where you get to "know' the other students. I absolutely love the convenience, though!
Best of luck to you!