A method is a special kind of subprogram belonging to a class.
An instance method is a method invoked with respect to an instance of a class. Instance methods are often used to examine or modify the state of a particular object. In Java and C++, constructors are instance methods which have the same name as their class. In typical implementations, instance methods take a hidden reference to the object they belong to.
In contrast to instance methods, a class method (a.k.a static method, shared method) can be invoked without reference to a particular object. These affect an entire class, not merely a particular instance of the class. A typical example of a class method would be one that keeps count of the number of created objects within a given class.
An abstract method is a method which has no implementation. It is used to make a place-holder to be overriden later.
An accessor method is a kind of method that is usually small, simple and provides the way to bypass access control. Although it introduces new dependency, use of the methods are preferable because they provide a certain layer.