grep expression,-or-block list

returns a new list containing those elements of list for which expression or block produces a true value, whereas

map expression,-or-block list

performs the expression or block for each element of list and returns the list of values the expression produced (i.e. one (*!) for each element supplied in the "input" list).

Both of these update the default variable $_ with the current list element before executing the expression or block. Examples:

my @sources = grep /\.c$/, glob '*'; # get only the files ending in .c
+ from a listing of the working directory

my @objects = map s/\.c$/\.o/, @sources; # get the list of object file
+s that a c compiler would make by default from the files now in @sour
+ces

my @virgins = map { my $c = $_;
                    $c =~ s/\.o$/\.c/;
                    system "cc $c";
                    $c } grep !(-f), @objects;
                # find and compile the uncompiled sources
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(*! Update: unless of course the expression generates more than one or zero values per iteration as johngg points out below)