Chocolate comes from the cacao bean which grows on a plant called Theobroma cacao. The beans are dried, roasted and ground into cocoa which is the basis of all chocolate. Chocolate liquor or cacao is simply ground-up whole cacao beans; cacao butter (nothing to do with dairy) is the natural fat from the bean; and cocoa or cacao solids is the solid part of the bean ground to a powder. Unsweetened chocolate contains 100 percent cacao content while semi-sweet has 52 to 62 percent cacao content. Converting unsweetened chocolate to semi-sweet is not only easy, but healthier because it has more anti-oxidants.
Read the recipe you are baking from and make note of the amount of chocolate required and whether it calls for semi-sweet cocoa powder or semi-sweet chocolate squares.
For cocoa powder, use 6 tablespoons unsweetened cocoa powder, 4 tablespoons butter and 7 tablespoons sugar in place of 1 cup (6 oz.) semi-sweet cocoa powder
For baking chocolate, use 1 square (1 oz.) of unsweetened chocolate plus 1 tablespoon of sugar in place of 1 square (1 oz.) of semi-sweet chocolate.
Do the math as necessary for the actual amount of chocolate needed in your recipe based on the conversions listed here.