There are a lot of articles on the web about model release, so do google model release , to find out more, but in short , you would normally be asked to sign a model release form after any commercial shoot as it reinforces the ownership rights of the photographer to use the images taken at the shoot.
If you are paid for a shoot, whether by money or in kind, then a bona fide professional photographer would always require this form.
If you are paying for the photos / cd then it would not necessarily be required, but the photographer may still request a release form , so that he can use the images to promote you or himself .