Wow. 8 posts, and a move, and NO ONE has answered the question he asked. Nice job guys...
The 4-3 defense is your standard run defense. Though it can be used for the pass as well, as you'll see in a minute, most teams use other formations for pass plays. The 4-3 is the most popular run defense, as it allows most of the offensive line to be covered, and gives your LB's or DB's a chance to "Blitz" or rush the QB if desired.
The 3-4 defense is actually VERY similar to a 4-3, but is used by coaches with better middle linebackers than linemen, OR coaches with a different philosophy on the game. Honestly, I'm not sure why else they use the 3-4, but some coaches have indicated it's a superior formation. Question is, did any of those coaches win a SuperBowl?

In any event, it's basically a run defense as well.
The Nickle formation is one of two "pass" defenses. You use the nickle typically when you think there a good chance the offense will pass. Most of the DB's are back to cover receivers, and only two LB's are left to watch the middle of the field, or try and stop a run. This exposes weaker defenses, leaving them vulerable to big runs on occasion. But I find that with good LB's... you can effectively stop the run despite the lack of help in the backfield. But generally use this if your THINK your opponent could heave a long one.
The Dime formation is the other main "pass" defense, and for when you basically KNOW your opponent is going deep. It leaves a SINGLE linebacker (LB) in the middle of the field, brings in nearly every corner and safety you have, and you get ready for the ball to fly. With this defense, you really have given up on stopping the run most of the time. Since your MLB is covering the middle of the field and any receiver that comes his way. But if he's going deep, you SHOULD have a CHANCE to stop him.
There are other defenses like the 4-4. Non-traditional D's that are heavy toward stopping the run if you KNOW that's what the other guy is doing, etc. But I stick with the basic 4 package, and have great success against the PC. How will that stick up against HUMAN compitition? Only time will tell.
Best advice I can offer is keep playing. The practice mode is great for letting you get used to the passing. The game is VERY quick, and takes a bit to get used to. But mix up your running plays and pass plays... too many passes will lead to interceptions... and you should get used to the speed of the game soon. Personally, I still sometimes find myself double pumping when trying to throw a hail mary late in a game... but that's just getting used to the passing system. We'll get there.
