Chain mail is an ancient form of body armor made of a series of overlapping links (or "chains") of metal either sewn onto a backing, like an article of clothing such as a jerkin or doublet, or as a standalone article. However, clothing is typically worn underneath such an article to prevent irritation and chafing.
Originally seen as a staple of Norman and Saxon armies, chain mail is a more cost and time efficient form of protection against some forms of assault. Considering the time and skill necessary to forge an entire suit of plate mail armor, most foot soldiers of these times were dressed in suits of chain mail. Add to the fact that repairs to such mail are easily made in a portable forge, and the reasoning behind equipping thousands of soldiers with armor becomes evident.
Chain mail can be of just about any length, including a full head to midthigh variation that was very popular in medieval Europe. In game terms, chain mail typically means a chain shirt, possibly reaching to just below the waistline. Chain mail is proof against most cutting weapons, but provides very little protection against blunt trauma. Also, thin blades, such as stilettos, can easily penetrate the space between links. Also, chain mail is used to cover gaps in plate armor, and in some cases, the plates themselves are simply stapled to preexisting suits of chain mail. Size restrictions should be considered important. Chain mail for a halfling-sized character would barely cover the thigh of your average human.