Okay, here is a breakdown of the Nickel defense. I'm doing this from memory, so don't crucify me.
Quarters Man: Here you have the 4 DBs playing zone and both the NB and the LBs playing man. The MLB is in the center of the field as suppose to Cover 4 where he will be on the far side of the defensive formation. You might also find that your OLB is closer to the LOS in Quarters than in Cover 4. Hence, it's easier to be jammed at the LOS and your WR gets picked up sooner. Here I attack between the seams and in the middle 1/3 of the field.
Inside Blitz: Here all of the DBs (including the NB) are in man coverage and the two free LBs are extra blitzers. The blitz comes from up the gut and off tackle. Find your hot reads and hit your crossing routes.
Man Zone: Here your corners and the FS play zone. Your FS will slide to the middle 1/3 of the field. If I remember correctly, the SS plays man on the backfield, but you may want to check into that. Your OLB plays man on your TE. Your MLB drops into zone coverage. Once again, I attack the seams, but you deep left and right side of the field are WR friendly.
Man: This is pretty much self-explanatory. You have deep safety help to either side of the field.
Man Under: This probably requires a little explaining. If you want to make it more difficult for a C to double-team your DTs, then shift your D-line "Over" or "Under" the O-line. This refers to the placement of your personnel. In an Over defense, your men are shifted to the strong side of the formation, and you have their T between two of your men. In an Under defense, your men are shifted weak side. Once again, their T is between two of your men. Neither is better than the other is, because they are both situational. In Man Under, your D-line will be shifted Under and your MLB plays man. Otherwise, it's exactly the same as Man Zone.
Silver: This is a double off tackle blitz. The MLB plays man, so do your corners and your safeties. What you want to do is hit your hot reads on streak patterns. If you don't have one a hot read, then audible and get you one. If you're on defense, put speedsters on the outside and flatten the QB.
Under 2 Deep: This is probably the best full zone that Madden has to offer. Your CBs take away both corners and your LBs provide overlapping coverage. Your MLB starts off shifted strong side. Your safeties play the deep corners. You're probably best off flooding the zones, hitting the seams, or attacking the middle of the field.
Under Smoke: Here your corners, safeties, and OLB are locked in man coverage. Your NB and MLB both blitz off tackle. (Your MLB starts off shifted strong side and will loop around your LE before rushing the QB.) If you're on offense, roll your TE. He will always be open.
Under 2 Man: Your corners, NB, & LBs are all locked in man coverage. Your MLB shifts strong side to get an early release on his cover man. The safeties provide deep corner support. Your D-line is shifted Under (see: Man Under).
Under St: This is an Under shifted D-line (see: Man Under) with full zone and both safeties shifting to the strong side of the field. Your MLB starts off shifted strong side. When on offense, flood the weak side.
Robber: Each safety does a cross for deep corner support. Your remaining DBs and LBs are locked in man coverage with your MLB shifted strong side.
Over St: This is a full zone. Here your D-line is shifted Over (see: Man Under) and your safeties shift to the strong side of the field. Your MLB starts off shifted weak side.
Over Weak: This is a full zone, which is exactly the same as Over St, but your safeties shift to the weak side of the field. Your MLB remains shifted weak side.
Cover 4: Here your two corners and safeties play deep zone coverage. The MLB plays man, but he is shifted to the weak side of the formation. You may want to re-read Quarters Man for further information.
Double Slot: In DBL Slot, your corners and FS are in man coverage. Your SS rolls to the middle of the field for lone deep coverage.
Strong Zone: This formation is identical to weak side Under 2 Deep with strong side man coverage. Your weak side DB cuts off the corner route.
Weak Zone: This is exactly the opposite of Strong Zone, but here your strong side DB cuts of the corner route.