Pousse Cafe is a French phrase that means "push the coffee" and refers to cordials (and similar drinks) that are served after the dinner along with coffee. These dessert drinks were introduced in New Orleans in the 1840's and became popular throughout by the early 1900s. The drinks are colorful and served in a tall thin glass (which also has the same name). Each layer is (in theory) meant to complement the next layer.
The drink is mostly for show and as such should use the glass that is tall and thin with a slightly wider opening than the goblet itself. The glass should be clean and dry for the drink to be poured - even the slightest drop of water may cause the interface between two liquors to be disrupted and mix.
Three layer drinks are not that bad to make may look nice and actually be drinkable (one example that I've had was peach vodka, cranberry juice, peach schnapps, clear, purple, clear layering). More complex
drinks are more difficult to make and become annoying for the bartender. Ordering a seven layer pousse cafe is akin to asking the band to play free bird.
To get the drink to look correct, it often uses some rather odd liquors such as green or yellow chartreuse (herbal flavors). Yellow chartreuse sits between chambourd (purple berry flavor) and goldwasser (clear with gold flecks cinnamon) and Green chartreuse sits between Grand Marnier (brown orange flavor) and brandy (brown brandy flavor). Some times the very bottom layer is that a raw egg.
For those of you who enjoy tormenting bartenders (not a good idea) or making your own, the following is a list of well known Pousse-Cafes. Others can be made consulting a table of liquor specific weights. It is possible to force two that would otherwise mix some by chilling a liquor to increase its viscosity.
http://cocktails.about.com/library/weekly/aa023100a.htm
http://hotwired.lycos.com/cocktail/97/15/alchemist.html