Unsweetened chocolate, as its name indicates, contains absolutely no added sugar, while bittersweet chocolate has anywhere from about 10% to 50% sugar. Because of the difference in sugar content, these two types of chocolate are not interchangeable in recipes.
The good news is this: bittersweet and semisweet are very similar. Bittersweet chocolate is often now labeled "dark chocolate" and clearly lists the percentage of chocolate. That percentage tells you how sweet the chocolate will be: chocolate labeled "70% chocolate" contains 30% sugar, "60% chocolate" contains 40% sugar, and so on. Semisweet chocolate tends to be higher in sugar than bittersweet or dark chocolate, but there can be overlap.
The bottom line: if your recipe calls for bittersweet chocolate, you can use dark or semisweet, and when using semisweet, you can go just a little light on the sugar. For more chocolate tips and recipes using all types of chocolate, see our collection of Chocolate Recipes.