What is the difference between Parameters and Variables in Informatica

Mapping Parameters

A mapping parameter represents a constant value that you can define before running a session. A mapping parameter retains the same value throughout the entire session.

When you use a mapping parameter, you declare and use the parameter in a mapping or mapplet. Then define the value of the parameter in a parameter file for the session. During the session, the Informatica Server evaluates all references to the parameter to that value.

Mapping Variables

Unlike a mapping parameter, a mapping variable represents a value that can change through the session. The Informatica Server saves the value of a mapping variable to the repository at the end of each successful session run and uses that value the next time you run the session.

Local variable

You can use local that you create within a mapping in any transformation expression. For example, if you use a complex tax calculation throughout a mapping, you might want to write the expression once and designate it. you thereby increase performance since the Informatica Server performs the calculation only once. Local are especially useful when used with stored procedure expressions to capture multiple return values. 

System variable  
These are constant through out the mapping and cannot be changed.

Eg.$$SessStartTime