Unsweetened chocolate contains no sugar and so is about 99% chocolate liquor. It's extremely bitter and cannot be used interchangeably with semisweet or bittersweet chocolate.
Chocolate for baking is manufactured in squares, thin bars, and thick blocks. Thin bars are convenient to store and can be easier to chop than blocks, which take a bit of elbow grease to knock apart (but are better for making decorative shavings and curls).
Some better quality chocolates only come in huge blocks, which are great for pros but may be too big a quantity for home cooks. Luckily, specialty stores often sell smaller chunks of these blocks wrapped in plastic.
Chocolate will keep for a year at room temperature, if kept below 70°F. Wrap it in a few layers of plastic wrap to keep it as airtight as possible and put it in a dark cupboard, away from strong-smelling foods. (Chocolate, like butter, will absorb strong aromas.)
You can store chocolate in the refrigerator or freezer, but a moist environment isn't ideal. If you do chill your chocolate, bring it to room temperature while still wrapped to prevent condensation from forming, as any water on the chocolate can interfere with its ability to melt smoothly.
If your chocolate has a white film on it (also called bloom), it means it has either been stored too warm, causing the cocoa butter to separate out. Though the chocolate may not have the same texture eaten straight, it will work just fine in baking.