Card marking is the process of altering playing cards  such that the suit, rank or both are only apparent to the person marking the cards (or potentially another conspirator), usually for the purpose of cheating at cards by card sharps. To be effective, the distinguishing mark or marks must be done on the obverse side, which are normally uniform.

Card marking can sometimes also be called as card bending; it is considered to be the easiest way to mark probably.

Marked cards are often used to cheat when gambling or for card tricks, though many casinos, particularly those in Las Vegas, punch holes through the middle of cards they sell to tourists to prevent cheaters from returning to the game tables after buying the cards and then slipping the favorable cards into their hands when playing.

Historically, the first attempts to mark playing cards involved bends, crimps and tiny bumps, which are called blisters (and resemble the Braille script). Later, when the first designs appeared on the backs of playing cards, cheats began altering the designs on the backs of cards. Hustlers have tried using various inks, pigments, scratches etc. to add or remove lines or patterns from the back of the card design. Some varieties of card marking include block-out work, cut-out work, scroll work, shade work, tint work, etc. Here is a simple example of how block-out work can be done.

Source: Wikipedia - Marked Cards