Hacker is a term used to describe different types of computer experts. The media and the general populace typically use the term to mean "computer criminal"; however, in many computer subcultures it simply means "clever programmer", with no connotation of computer security skill. It is also sometimes extended to mean any kind of expert, especially one who has particularly detailed knowledge or cleverly circumvents limits.
Anyway, back to the site.
It can be accessed from this address: www.hackthissite.org
The main purpose of this site is to introduce hacker wannabes (again, talking about computer experts) to to complexity of Internet and the variety of options available to a ordinary user.
First of all, there are many challenges.
Next, there are so call Realistic Missions. These are harder to finish, because there are some realistic elements. It is a lot harder to hack (this time I mean hack) a real system than a one which is meant to be hacked. Here you can learn about SQL Injections and other ways to fool the server.
Furthermore, there are Encryption, Programming, Permanent Programming and Application Challenges. I have nothing to say about them, because I haven't tried them yet.
When we move further on, we can see that there are a lot of useful articles, that can help you increase you knowledge. They vary from mission tutorials to complicated techniques.
I haven't mentioned it, but in order to play these challenges, you must register first. This is good, because for every solved challenge you get a certain amount of points. This is where the fun starts. They even give you point to hack their site! I think only two people managed to do this, but there is still time for you to join them
Next, there are a few ways of communication with other players. There is a forum, where you can find hints, but not solutions, for all of the challenges, and also talk to other people about other things. Also, you can talk to them by using IRC chat, which might be even more useful if you want to ask for help.
To sum up, the site is great! Challenges get harder all the time, you can learn really much, and that is not all you can do. There are some really nice people out there to talk to. Negative stuff - well, someone can become a really evil hacker by using this site.
My opinion: 9/10