Amateurs is one of the most abused terms in the adult online business. Technically, Amateur should refer to a girl (or guy) who is running a site not for profit but for fun. Basically, they site runs not because the girl wants to get paid to get naked, but rather just because she wants to get naked.
The reality is that at one level or another, almost all amateur sites are either profit motivated, or are at least run with the intent of making some income. Many girls (or guys or even couples) run personal websites mostly for the thrill of showing off, being seen, etc... and charge only minimal membership fees (if anything at all) to allow people into their sites. Usually this money is kept to pay for computer services like hosting and the domain name they use.
There is a second set of "amateurs" that are a bit more profit motivated. Individual girls that run sites with the intent of making money. It is, in a sense, sort of like becoming a stripper but safe at home or at their boyfriends house or whatever. These girls are sometimes called "indy" amateurs.
The third set is the "group of amateurs". This would be girls that would most likely be in the previous set, but have grouped up to offer more to their members. They often share live cam shows, so that members of their sites can see other cam shows, or by appearing together and offering the content in both sites. These girls often work together to plan shoots share expenses, and pretty much make things better for everyone involved. In my personal opinion, these sites can often offer the best ratio of cost to content or action, some of the better groupings have cam shows every day, a nice selection of girls, and a friendly atmosphere. Very nice stuff.
Finally, there is what is known as a corporate or paid amateur. These are girls that sign longer term contracts with website developers to be the name or the face in the front of a website that is administered as part of a larger, more corporate structured program. The program is very much oriented towards marketing the girls image and likeness, and getting more visitors to the sites to find more members. You will often find the best quality images, videos, and the best presented girls on this type of website. They will often also have appearances from other girls inside the group, and having nice promovideos and such. These sites will sometimes be a little more expensive to join, but the image quality and video quality is usually second to none.
You can find out more about amateurs by check out out category:amateurs section.