Sometimes if you’re trying to sell photos you won’t legally be allowed to because you’ve photographed someone and their face is clear and identifiable in the image. A model release form is a contract between you and that person allowing you to sell that image.
Here’s a generic model release form for download that could be used for most situations. The language might be a bit frightening but that paragraph of text is so dense I suspect many people won’t read through it properly.
There are plenty more release forms to download so you don’t have any excuse to be caught out without one! The istockphoto release form looks to be more complete at first glance, but you would have to remove their logo and change the wording slightly if you want to use it for other agencies or stock sites.