Unsweetened chocolate is chocolate in one of its simplest forms, a solid chocolate made with just cocoa solids and cocoa butter. The natural fat content of a cacao bean is 52-55%, which is typically the amount of fat (cocoa butter) found in unsweetened chocolate. The exact ratio of cocoa solids to cocoa butter will vary slightly from producer to producer, with smoother unsweetened or plain chocolates having slightly more cocoa butter in them. This mixture of cocoa solids and cocoa butter, when it is still liquid during the production of chocolate, is known as chocolate liquor. To produce other chocolates, this liquor can be mixed with milk solids, sugar, vanilla and other ingredients to create a wide range of milk and dark chocolates.
Unsweetened chocolate is not a popular choice for eating, since it has a very bitter taste to it, but it is a common ingredient when it comes to baking and cooking because it makes it easy to add a strong chocolate flavor to a recipe. It is less commonly seen today, as there is a wide range of quality dark chocolates to choose from (whereas a few decades ago the choices were slim), but it is still a staple in many pantries. Recipes like chocolate cakes, brownies and cookies will be where you are most likely to see unsweetened chocolate pop up, but occasionally they will also be used in ice creams and puddings where you can easily add extra sugar to sweeten the recipe to taste.
You can substitute unsweetened chocolate into a recipe for dark chocolate by slightly increasing the sugar in your recipe, although you will probably find that this is completely unnecessary if you are only adding in a small amount of chocolate. Similarly, you can substitute dark chocolate for unsweetened chocolate by reducing the sugar very slightly. You probably don’t want to substitute unsweetened chocolate for chocolate chips in any recipes where you get big pieces of chocolate, since the bitterness of unsweetened chocolate probably won’t give you the finished flavor profile you were looking for.