In a nutshell, it's a library that provides attribute-based enforcement of business rules on your entity objects. It comes with a bunch of pre-defined attributes to handle common scenarios (e.g. a field is required, a field should have a maximum length, a field should match a certain regex, etc.). It also easily allows for adding new, custom rules.

The thing I like best about this project is that it doesn't force you into descending your classes from some super class. You can do that if you want, but it will also work if you want to leave your classes alone. All in all, I would highly recommend looking into using EViL in your .NET applications.