If you had bought an original cartridge for Pokemon Diamond and Pearl, I'm afraid the following content may be not suitable for you to view as ruining your mood and craft a deep scar inside your head skull.
PokeSav, also known as Pokemon Cheat Editor, is a program developed by a hacker from Nintendo. It's just a rumor about the developer and I had not yet gain extra information about it.
Before you trying to use PokeSav, you will need to extract your Pokemon D/P .sav file to your computer, then open it by using PokeSav. In other words, unless you're using flashcart to play Pokemon D/P, you are unable to use PokeSav to modify your pokemon data.
PokeSav seems to be gaining popularity over the Internet because of the fact that you can edit EVERYTHING regarding your .sav file. For example, you can modify your pokemon IV, EV, nature, moveset, captured date, captured place, obtained ribbons and so many more. It's not only limit to those though, you can modify your own trainer name, money and even event flags. Nothing is impossible to modify with PokeSav!
However, PokeSav seems to be causing an uproar anywhere in the Net. The arguing is even worse among original Pokemon D/P cartridge user as they cannot seems to tolerate with such cheating device. About 40% of the original D/P user claimed that they're afraid to trade at WFC via Wi-Fi as they scared to be trading with a hacked pokemon.
To cope with this problem, Nintendo had took step, minor step to be precise. User can only trade LEGIT pokemon among each other. Legit is pokemon that had correct information about captured location, trainer information and correct moveset that can be learned by it (egg move included). In other words, you cannot trade Raichu with Hydro Pump as one of it's moveset with another user as it will be considered as NON-LEGIT.
When Wi-Fi battling, you can still use those Pokemon which you modified by using PokeSav with one condition - All moveset, pokemon information are LEGIT. (Thanks Phx_Cyp for the info)