Is there any difference in Procedure or Function in a program?

In addition to what the others have said, there is also a difference with respect to how they treat the paramters they are passed. When you call a procedure like this:

DO MyProc WITH MyParam

by default, the parameters are passed by reference. This means that any changes made to them in the called procedure will be seen in the calling procedure. Of course, you can get around this and pass the parameters by value like this:

DO MyProc WITH ( MyParam )

On the other hand, when you call a function, parameters, by default, are passed by value. This means that any changes made in the called function will not be seen in the calling program. Of cource, you can always get around this and explicitly pass the parameters to the function by reference:

luRetVal = CallMyFunction( @param1, @param2 )