Creme de Cacao is a chocolate liqueur flavored with cacao beans. Other flavorings including vanilla are sometimes added. You will usually find an alcohol strength between 30 and 50 proof.
Although the French word creme is part of it's name, Creme de Cacao does not have any cream in it. There is a certain richness in the flavor profile - perhaps that's the reason for the "creme".
Creme de Cacao liqueur is found in a white (clear) version and a dark (or brown) version. The dark version is colored with caramel coloring, not chocolate. The only difference between the two liqueurs is the color - the flavor is the same. Drink recipes will usually specify one or the other to achieve a certain color (or lack thereof) in a drink. If the color is not specified then use white Creme de Cacao.
If you need a substitute then you are pretty much out of luck. First thing is use the other color if you have it. Failing that, it is tough to come up with a substitute. Creme de Cacao has a cleaner, less sweet flavor profile than most other chocolate liqueurs. There are subtle cacao bean flavors present that can add to a cocktail without taking over.
So you should probably stock a bottle since there is no good substitute. If you want to keep only one color around choose white. Creme de Cacao will keep a good long time as long as you keep the head space in the bottle to a minimum (decant into a smaller bottle once you have used half of it to prevent oxidation).
I use Bols Creme de Cacao and I have been very happy with it. Bols has a nice, clean, chocolate flavor and is a high enough proof to avoid any syrupy consistency. I have read good things about Marie Brizard as well.
Creme de Cacao is the "secret" ingredient in some Tiki drinks. Some Zombie recipes use it in small amounts (though Donn the Beachcomber did not use it). You will also find Creme de Cacao used as a modifier in some cocktails - I see it all over the place in Jones' Complete Barguide.
But the most common place you will find this liqueur is in liquor/liqueur or cream-based after dinner drinks such as the Brandy Alexander or the Grasshopper. If you want to make a drier, leaner Chocolate Martini try using Creme de Cacao as your sweetener instead of a cream-based chocolate liqueur.
Do you have a brand that you prefer?