Creme de Menthe is a minty flavored liqueur available in green or clear. Creme de Menthe is used for a variety of desserts in addition to being used as a liqueur or in mixed drinks.
While it may vary per brand the alcohol content is typically about 25%. A classic cocktail made with this liqueur is the Grasshopper. The drink dates back to the early 1900's and is made with equal parts green crème de menthe, white crème de cacao, and cream.
Back in the 1960's and 1970's some "nicer" restaurants served Creme de Menthe over ice cream as a dessert.
There are some decent substitutes for creme de menthe, depending on what your use might be. Per 3 tablespoons needed you can substitute: