Heraldry
Heraldry is the name for using heraldic devices as a sort of I.D. It's only called heraldry because heralds were the people who, in a sense, read these heraldic devices. When heraldry spread, however, this art was then taught and made part of the squire's responsibility.
The main heraldic device was the coat of arms (escutcheon). A coat of arms was a knight's shield with a design, although all knights had a coat of arms, some of them also had banners. This was so because nobody would be able to tell the knights from each other if they were all covered in armor. There were only seven colors used on a medieval coat of arms: yellow, white, red, blue, green, black, or purple. These seven colors were used to create the charges (the designs on the coat of arms). Such designs were animals, plants, objects, bands, chevrons, stripes, crosses, stars, etc. Most knights often chose animals that hunted and were vicious, like gryphons, lions, dragons, etc. A lot of the artists who designed the knights coat of arms often used imaginary animals like the basilisk, dragon, unicorn, etc. If a herald was "reading" a coat of arms with an animal on it, he would have to take its stance, position, and color into consideration. Its position was just where it was on the shield, its color was its color, and its stances could be passant, couchant, rampant, or statant. These stances meant walking with its front paws raised (passant), lying down (couchant), rearing up (rampant), or standing up (statant).
Over time, it became necessary to have each noble son add another charge to the original family coat of arms. So the eldest son would add a label, the second son would add a crescent, the third son would add a mullet (a 5 pointed star), the fourth would add a martlet (a bird drawn without feet), etc.