There are 281 total Madden Cards this time around. Player cards give that player a boost for a specified amount of time (based on the type of card--bronze, silver or gold). Playing a Coach card boosts your team awareness for a specified amount of time. Stadium Cards unlock stadia for exhibition mode. Historic Team cards open those squads for exhibition, and Cheerleader/Pump Up Crowd Cards give your team a temporary boost. Here are the card categories, and how to gain the locked Cards within each:

The following cards are unlocked by completing Mini-Camp challenges (the rest can be gotten by purchasing packs of Cards with Tokens):

#2: Unlock by getting gold in the All-Madden Chase and Tackle drill.
#30: Unlock by getting gold in the All-Madden Coffin Corner drill.
#38: Unlock by getting gold in the All-Madden Ground Attack drill.
#42: Unlock by getting gold in the All-Madden Precision Passing drill.
#52: Unlock by getting gold in the All-Madden Swat Ball drill.
#99: Unlock by getting gold in the All-Madden Trench Fight drill.
#104: Unlock by getting gold in the All-Madden Clutch Kicking drill.
#147: Unlock by getting gold in the All-Madden Pocket Presence drill.

Coach Cards #156-187

These Cards, when used, raise your team's awareness for a specified amount of time.
Cheat Cards, when played, produce special game conditions for a specific amount of time, based on whether they are Bronze, Silver or Gold. Certain ones need to be unlock via Mini-Camp drills. They are:

#195: Unlock by getting gold in the Ground Attack drill.
#197: Unlock by getting gold in the Clutch Kicking drill.
#199: Unlock by getting gold in the Swat Ball drill.
#208: Unlock by getting gold in the Pocket Presence drill.
#209: Unlock by getting gold in the Trench Fight drill.
#210: Unlock by getting gold in the Precision Passing drill.
#211: Unlock by getting gold in the Coffin Corner drill.

Stadium Cards: #220-224

Unlock these Cards, and you'll unlock the stadiums, and make them available for exhibition games.
Unlock these Cards to make the Historic Teams available at the team select screen.
These are unlocked by completing each team's Situation in Mini-Camp mode, with the exception of these teams:

#251: Cincinatti Bengals
#260: Dallas Cowboys
#264: San Francisco 49ers
#273: St. Louis Rams

Complete a challenge in Mini-Camp to open that same skill challenge on the higher difficulty setting.

To enter these codes, go to My Madden at the menu, select My Madden, then go to Madden Codes. Enter the case-sensitive code to unlock the corresponding Card:

P67E1I: TJ Duckett (Gold)