They are both filters working on a list. For each element of the list, the first argument of grep or map is executed (this argument being either an expression or a block). Just before the execution, $_ is set the element being processed.

grep and map differ in the return values. The former will always returns a subset of the original list (the entire list is a subset as well). An element of the original list will be returned if the expression (or block) returned a true value when that element was processed. The return value of map is a list consisting of the actual return values of the expression or block.

Abigail