Crackme is a program designed for and by crackers to provide some test ground for their abilities. It can be used to:
The idea of crackme is quite close to legal 'hacking games' proposed on many websites. Writing crackmes is not so popular today as good protection became domain of commercial solutions.
Typical crackme consists of two textboxes: username and password (resembling a typical 'register' form) and you are supposed to break the authentication process. This is in most cases done by disassembling crackme file and analysing the password checking function.
To complete the task in a good style you have to obey some basic rules. Main rule is that you can't make changes to file itself. You have to guess / break / get from code the username and/or password.
And that's the real FUN. After breaking some of them you can really call yourself an expert!