OnSexCam is one of the newest, and best, adult chat sites. Since they're are so new they are not just as established, as eg. LiveJasmin (another great live webcam chat site), but never the less the performers they do have, are top notch!
Once you enter the site, you see a list of all the currently available performers with preview photos. From this list, you choose the girls you would like to get intime with, and a Java applet will load.
The applet let's you view the performer directly from the performers webcam. Since what you see there, is a free for all feed, the quality is limited. More about that later.
Now you can chat directly with the performer, making her do excatly what you desire!

There is, however, limits on what you get for free. While chatting with the performer for free, you'll also see the text of the other chatters, and they can see what you write. And stated above, the quality of the video stream is also limited.
To chat private with the performer, with high quality video, you must pay. What you do is create an account. Once the account has been created, you can insert money into it. Now you can go into, what is called private mode. In private mode, you're all alone with the performer.