Photo © Chance Agrella
Though commonly called a 'goatee', the facial hair that is below the mouth and includes a mustache is not a goatee, but technically a 'Van Dyke'. The common misuage of the name has made the term 'Goatee' an accepted name for the Van Dyke style, though hair professionals and other familiar with both styles and terms tend to differentiate between the two.
If you are asking for help with shaving or styling it may be important to keep the difference in mind.
I personally prefer the look of a Van Dyke over that of a Goatee, however, I have seen some men who were able to pull it off.
With that in mind, here are some quick links to info about Goatees and Van Dykes.
How to Grow a Goatee (with pics of a Van Dyke)
Goatee Style (blog)
Goatee Gallery (pics of Van Dykes)
Printable Van Dyke Beard
The Goatee by Wikipedia (with a picture of a goatee and a Van Dyke)
Facial Hair Types Chart