The savfile is the full record of your game, it contains your name, your ID's, all of your pokes, your progress thus far etc etc etc, and the pokesav is a program that allows the edition of such file and it's contents, the most notorious edition is by each monster where you can edit its IV's, EV's, Nature, every aspect of it.
And is indeed higly popular, just to tell you, there is already a B/W version of it XD