A "parameter" is a placeholder. Parameter is variable in the declaration of function.
An "argument" is a position that is being considered. Argument is the actual value of this variable that gets passed to function. 


Think of the argument as the male part, making the parameter the female part. The argument goes into the parameter.

A parameter is usually used in definitions. An argument is usually used in invocations.
Or even simpler...
Arguments in !Parameters out !

Arguments are also called as actual parameters, as they are the actual values being passed.
Parameters are also called as formal parameters, as they are not original values, just the replication of the original values.