Types of Chocolate

Cacaoaka Cacao Nibs, Raw Cacao, Roasted Cacao, Ground Cacao
This is the cacao bean, minus the shell, and nothing else. You can buy cacao raw or roasted. Whole cacao is the whole bean, cacao nibs are crunched up pieces of bean, and ground cacao is powdered. Really the healthiest form of chocolate there is, cacao can sometimes be quite bitter.
Chocolate Liquor
This is the basis of all types of chocolate, formed by grinding cacao nibs into a smooth, liquid paste. Nothing is added, and it is not alcoholic. It naturally contains about 53% cocoa butter (fat).
Unsweetened Chocolateaka Chocolate, Baking Chocolate, Pure Chocolate, Bitter Chocolate
Chocolate liquor that has been allowed to cool and harden. It is used for baking and to make other types of chocolate. Many bakers prefer this type of chocolate for baking because they have more control over the flavor and sweetness.

Bittersweet Chocolateaka Semisweet Chocolate, Dark Chocolate
Contains at least 35% chocolate liquor, plus cocoa butter and sugar in varying amounts. There is no technical difference between bittersweet and semisweet types of chocolate, and they are often referred to as "dark." Note that there is such a thing as "bittersweet (or semisweet) baking chocolate," which is sweetened cocoa liquor without the added cocoa butter.
Sweet Chocolate
Contains at least 15% chocolate liquor, plus cocoa butter and sugar in varying amounts. Some people mistakenly refer to this as "bittersweet."
Milk Chocolate
Contains at least 10% chocolate liquor, plus cocoa butter and sugar in varying amounts, and at least 12% milk (milk, cream, milk powder, etc).
White Chocolate
White chocolate is not technically one of the types of chocolate because it does not contain any chocolate liquor. It must contain at least 20% cocoa butter and 14% milk, plus sugar in varying amounts.
Cocoaaka Cocoa Powder, Unsweetened Cocoa Powder, Unsweetened Cocoa
Is made by slamming chocolate liquor with a hydraulic press to expel the fat, i.e. the cocoa butter. What's left is allowed to harden, and then it is crushed into a powder. There is roughly 10-20% fat remaining in the powder. Cocoa powder is often used in low fat cooking because it retains the chocolate flavor but has much of the fat removed.
"Dutched" cocoa is formed by washing cocoa powder with an alkali solution of potassium carbonate. This darkens the color and neutralizes the acidity of the powder. Very alkalized cocoa is called black cocoa, which gives Oreos their unique look.
How do you know which to use in a recipe? Most American recipes use plain cocoa powder - good ol' Hershey's is plain cocoa. If a recipe needs Dutch cocoa, it will usually specify it. In general, regular cocoa is used in recipes with baking soda (which is alkaline), and Dutch cocoa is used in recipes with baking powder (which is acidic).
Ground Chocolateaka Powdered Chocolate
Not to be confused with cocoa powder, this is regular eating chocolate that's been ground to make a powder. It is generally used for making drinks, and should not be used in place of unsweetened cocoa powder in recipes.
Baking Chocolateaka Baker's chocolate (Baker's is also a brand name)
What is up with baking chocolate? Does it have sugar added or not? ARRRRG! Well, here's the thing: although the FDA sets the guidelines for what types of chocolate can be labeled "unsweetened, bittersweet, semisweet, milk, and white," they don't specify what can be labeled baking chocolate.
You can find all of the following types of chocolate labeled "baking chocolate":
1) unsweetened chocolate2) bittersweet baking chocolate (chocolate liquor + sugar, but no cocoa butter added)3) bittersweet chocolate (chocolate liquor + sugar + cocoa butter) Most chefs wouldn't consider this true baking chocolate because of the added cocoa butter, though you might find it labeled as such.4) baking-resistant chocolate, i.e. chocolate chips (bittersweet chocolate with less cocoa butter added, so that it won't melt easily)
So how do you know which to use? Hopefully your recipe specifies! In general, recipes will usually specify at least "unsweetened baking chocolate (#1 above)" or "bittersweet baking chocolate (#2 above)." Whether there's cocoa butter added or not probably isn't going to make or break your recipe.
One thing you should avoid, though, is using chocolate chips in place of other types of chocolate when the recipe calls for melting. The low cocoa butter content makes chips bad for melting.Chocolate Coatingaka Compound Chocolate Coating, Summer Coating, Chocolate Flavored Coating
These are vegetable fat-based coatings that contain sugar and some amount cocoa powder, chocolate liquor and/or cocoa butter for flavor. They are not true chocolate. The advantage to using them is that they typically do not "bloom" in high heat. They are best used in making chocolate decorations.
Couvertureaka Coating Chocolate
Chocolate coating or coating chocolate? Aaaaaaah! Couverture is the good stuff - usually some type of dark chocolate with extra cocoa butter added to make it melt nicely for enrobing (drizzling onto the outside of a chocolate confection). Because the high cocoa butter content (roughly 35-45%) makes it melt well, it is ideal for chocolate fountains, and usually no oil need be added.
Giandujaaka Gianduia, pronounced zhahn-DOO-yuh
Chocolate made with toasted hazelnuts ground into powder. It still has a smooth, chocolatey texture, but has the wonderful flavor of hazelnuts. An Italian or Swiss invention, depending on whom you believe. Who cares? It's yummy.
Single Bean Chocolateaka Origin Chocolate, Single Origin, Estate Chocolate, Grand Cru, Single Cru
In general, these types of chocolate that are made from a single type of bean that's grown in a specific region, or even a specific plantation. But not always. We might be talking about a bunch of types of beans all grown on the same plantation. Or a single bean from a bunch of different plantations in the same geographic region. Or heck, a blend of the finest of the same exact type of bean from locations around the globe. It's hard to say.
The point is the manufacturer is carefully selecting the beans to create a unique flavor, but some people argue this is a gimmick. After all, Hershey's selects its beans to create a unique flavor too! In general, however, these types of chocolate are of high quality.
Cocoa Butter
When chocolate liquor is pressed to expel the fat and make cocoa powder, the fat expelled is cocoa butter. Cocoa butter is added to chocolate liquor to make the type of chocolate we enjoy eating; it gives chocolate that smooth, melt-in-your-mouth texture we love.
Cocoa butter is also used in cosmetics and pharmaceuticals. Because it melts at about 97° F, it smoothes into the skin nicely. Also, it has healing properties and is resistant to spoilage.
Chocolatesaka Chocolate Candies, Truffles, Creams, Pralines, etc.
When people speak of "chocolates" in the plural, they are typically referring to chocolate candies, like truffles, chocolate creams, chocolate-covered nuts, and that sort of thing. "Chocolates" are candies made from other types of chocolate.
Chocolate Extract
Chocolate extract is a good way to add chocolate flavor to your cooking without adding fat, but the flavor can be a bit strong. It is made like vanilla extract; cacao beans are soaked in alcohol.
Chocolate Oil
Alas, there is no such thing as a chocolate oil. If you see chocolate oil, it's most likely a chocolate perfume oil, entirely manmade, and not for cooking.