What are Validation Annotations?

Data annotations are attributes you can find in System.ComponentModel.DataAnnotations namespace. These attributes provide server-side validation, and the framework also supports client-side validation when you use one of the attributes on a model property. You can use four attributes in the DataAnnotations namespace to cover common validation scenarios,
Required, String Length, Regular Expression, Range.

http://www.codeproject.com/Articles/639717/MVC-Interview-Questions-and-Answers-All-about-MVC