Generics are a new feature in version 2.0 of the C# language and the common language runtime (CLR). Generics introduce to the .NET Framework the concept of type parameters, which make it possible to design classes and methods that defer the specification of one or more types until the class or method is declared and instantiated by client code. For example, by using a generic type parameter T you can write a single class that other client code can use without incurring the cost or risk of runtime casts or boxing operations

Sample :-
// Declare the generic class
public class GenericList<T>
{
void Add(T input) { }
}
class TestGenericList
{
private class ExampleClass { }
static void Main()
{
// Declare a list of type int
GenericList<int> list1 = new GenericList<int>();

// Declare a list of type string
GenericList<string> list2 = new GenericList<string>();

// Declare a list of type ExampleClass
GenericList<ExampleClass> list3 = new GenericList<ExampleClass>();
}
}

See more help :-
http://www.csharpfriends.com/Articles/getArticle.aspx?articleID=370
http://www.c-sharpcorner.com/UploadFile/sdhar8po/GenericsInCSharp11152005055344AM/GenericsInCSharp.aspx
http://www.knowdotnet.com/articles/generics101.html