This is a question that comes up a lot in these forums, so I've put together a bit of a primer on the subject with links to relevant sites and previous discussions.
Let's start with the basics.
What is a model release?
A model release is basically a contract that says that you're allowed to to publish a photograph. It is something that you have the subject sign to show that they are ok with it. Read up on
Dan Heller's primer on the subject for more information.
So, what does this model release look like?
Model releases don't have to be complicated, but they can be, especially when people start requesting particular things (like a clause that says you can't photoshop the image). There have been several threads discussion the actual model release document.
Do I really need a model release?
That question is not nearly as clear as the first one. The answer is that it depends. The more
complete article by Dan Heller explains that there are
4 questions you should ask yourself. But there are several other things you should probably look at as well, including whether or not the image is being published, or if it's being published commercially.
This is one of those questions that has no easy answer unfortunately, so it requires a lot of thinking about your particular situation.
Are there any other situations where I need releases to take a picture?
Short answer: Yes.
Long Answer: Yes, and it depends on what or where.
Property releases are another form of release that you might need. You should be aware of what the local rules are though about trespassing and privacy and your
rights as a photographer.
So, if you're not totally confused by now, congrats. If you are, well, that's ok too, because lots of people have the same sort of questions. So, keep asking, and feel free to add any other knowledge about model releases you have to this thread.