tar does not compress the files -- the resulting tar file will be as large (or nearly as large) as the sum of the size of the files that are in the tar file.
And as the other two said, compress can be used to make that tar file smaller. It is native to the operating system and is the one you should use if you are sending a file to some other site and you don't know if they have zip or gzip.
And zip and gzip are shareware that do the same as compress, although in some cases, they can make a file smaller than using compress.