Difference between procedure and function

Difference between procedure and function.? 
Functions are named PL/SQL blocks that return a value and can be called with arguments procedure a named block that can be called with parameter. A procedure all is a PL/SQL statement by itself, while a Function call is called as part of an expression.

Which is more faster - IN or EXISTS? 
EXISTS is more faster than IN because EXISTS returns a Boolean value whereas IN returns a value.