A type can associate operations with that type or with each instance of that type. Such operations are called methods. A method is named, and has a signature (see §I.8.6.1) that specifies the allowable types for all of its arguments and for its return value, if any.
A method that is associated only with the type itself (as opposed to a particular instance of the type) is called a static method (see §I.8.4.3).
A method that is associated with an instance of the type is either an instance method or a virtual method (see §I.8.4.4). When they are invoked, instance and virtual methods are passed the instance on which this invocation is to operate (known as this or a this pointer).
The fundamental difference between an instance method and a virtual method is in how the implementation is located. An instance method is invoked by specifying a class and the instance method within that class. Except in the case of instance methods of generic types, the object passed as this can be null (a special value indicating that no instance is being specified) or an instance of any type that inherits (see §I.8.9.8) from the class that defines the method. A virtual method can also be called in this manner. This occurs, for example, when an implementation of a virtual method wishes to call the implementation supplied by its base class. The CTS allows this to be null inside the body of a virtual method.
[Rationale: Allowing a virtual method to be called with a non-virtual call eliminates the need for a "call super" instruction and allows version changes between virtual and non-virtual methods. It requires CIL generators to insert explicit tests for a null pointer if they don’t want the null this pointer to propagate to called methods. end rationale]
A virtual or instance method can also be called by a different mechanism, a virtual call. Any type that inherits from a type that defines a virtual method can provide its own implementation of that method (this is known as overriding, see §I.8.10.4). It is the exact type of the object (determined at runtime) that is used to decide which of the implementations to invoke.