They are 10 minikits per every stage of the main campaign, hidden in (almost) inaccessible places or related to specific tasks. During gameplay you can collect 150 minikits in total (15 missions x 10 collectibles). To obtain them, you must unlock free play mode first and choose the right hero with appropriate skills - they are some riddles, that only specific characters can resolve. If you have some problems with that, don't be worry: this guide explains everything very precisely.
To unlock free play mode, you must complete every mission in the main campaign - only then you will get access to extended character reel. To open it, hold down a change character button. Remember, that some characters must be bought first or even unlocked in the New York city area (for more information read the appropriate chapters of this guide).
Finding every minikit in the game does not provide you any specific reward, but you will get one gold brick and one comic book after collecting all minikits on the stage. Obtaining all collectables is also required to finish the game in 100%.
If you want to see your comic books, go to the Deadpool's chamber on the S.H.I.E.L.D. Helicarrier and use the white sheet hung on the right. To get there, you must exit the main room of the vehicle, using the door located on the left side. You will get access to this place after finishing the main campaign.