HFS is a much older standard; it stores data onto the disk in 16kb chunks... Thus, if you have a small text file, it'll take up 16kb no matter what.. HFS+ appeared in the middle, late 1990's (I think with OS 8)... It stores data in 4kb chunks... With all the small files contained in a modern Unix system like OS X, it's easy to see where you'll save a lot of disk space using it. The only real reason to format a drive in HFS is to make it compatable with older Macs, running system 7 or so - prolly pre PowerPC machines!
For all intents and purposes, this is similar to the FAT16, FAT32 difference in wintel...
See ya!