Deoxys is a unique, versatile, and powerful Pokemon. It's the only Pokemon that has four unique forms at the same evolutionary stage and it's the only Pokemon that has partnered with NASA (we hear Deoxys has a nasty addiction to Tang). Previously, if you were lucky enough to catch Deoxys, you were stuck with whatever form it was in when it fell to your mighty Pokeball. Thanks to a nifty enhancement in Diamond/Pearl, you can fully exploit the versatility of Deoxys to match your team's needs and your personal battle style.
There are four forms of Deoxys: normal, speed, attack, and defense. The forms are pretty self explanatory. If you don't understand them then we're mystified as to how you had the wits to catch Deoxys in the first place. To change the forms of this Pokemon, head on over to Veilstone City. On the eastern side of town, you'll see three pits with meteorites. If you have Deoxys in your party and examine one of the meteorites, it'll change forms. The northern-most meteorite is for speed and the middle one is for normal. There are actually two meteorites in the southern-most pit; the left one is for defense and the right one is for attack.