WCF Service InstanceContextMode and ConcurrencyMode combinations

What does the InstanceContextMode and ConcurrencyMode attributes in a wcf service do?A short MSDN description of this describes it as follows: “A session is a correlation of all messages sent between two endpoints. Instancing refers to controlling the lifetime of user-defined service objects and their related InstanceContext objects. Concurrency is the term given to the control of the number of