A common is like it I see small solar system body. When it is passing the sun it warms and begins to release gases in process called outgassing. That is the best I could do in my words describing a comment.
A comet is an icy, small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process called outgassing. This produces a visible atmosphere or coma, and sometimes also a tail.
