The Milky Way is a barred-spiral galaxy approximately 250-300 thousand light-years in circumference and containing 200-400 billion stars. The Sol system, containing the planet Earth, is located in one of the Milky Way's spiral arms.
The Milky Way is often used as a model when estimating interstellar travel distances. For instance, if a trip is described as going from the "core" of a galaxy to the "rim" of the galaxy, the distance can be estimated to be a significant fraction of the 125,000 light-year radius of the Milky Way.
In Star Trek, Federation maps of the galaxy divide it into four quadrants.