Shirts versus Skins (or 'Shirts and Skins') is a common form of denoting team affiliations in an informal sports game; typically when played on a public court or park. The practice involves the members of one team wearing shirts while the other does not. This is used in the absence of uniforms.
This simple form of recognition is a regular sight in streetball, street football, ultimate, and other pickup games. The identification by manner of dress negates the need to remember the division of players among friends, or to learn the faces of strangers in a pickup game. The system is mostly for men’s teams, owing to the taboo in many western cultures against the display of the female breast.
As a gay, chubby, nonathletic shy kid - simply the phrase "Shirts and Skins" sent chills down my spin in high school gym class.
However, these gentlemen are helping to ease the pain.