The brass rat is the name of the class ring for MIT students. Each year, the rat is redesigned by a student committee, but there are several things that remain consistent through the years.

The Face
The face of the rat has on it a beaver (the MIT mascot), in profile. Undergraduates wear their rat so that the rear end of the beaver faces them; alumni/ae wear the rat with the rear end facing away (As a student, the beaver shits on you; as an alum, it shits on the rest of the world). Hidden in the face are usually the letters MIT, and the class year.
The Skyline
The sides of the ring normal to the finger's axis contain the skyline of Boston on one side, and Cambridge on the other. Boston is on the "bottom" (ass end) of the ring), so it faces undergraduates (who look out at Boston), while Cambridge is looked at by alums. (Graduate student versions of the brass rat typically do not have skylines).
Mens et Manus
One side of the ring has Mens et Manus (from the MIT seal), usually with MIT in large letters above them.
Great Dome
On the face opposite Mens et Manus the ring has the Great Dome, facing onto Killian Court. Inscribed onto the dome one usually finds IHTFP. The class year is also inscribed on this face somewhere.

Examples of some hidden features of the 1997 brass rat:

The brass rat is traditionally made of gold; however, many years also have a steel version, which is known as a Stainless Steel Rat.