Semisweet chocolate and bittersweet chocolate are both subcategories of sweet chocolate. While both must also contain a minimum of 35% chocolate liquor, semisweet chocolate will contain less sugar than sweet chocolate, and bittersweet even less.
Read the entry on semisweet chocolate chips for information specifically about semisweet chocolate chips.
Substitute bittersweet chocolate.
Chop chocolate with a heavy chef's knife and a stable wooden cutting board. Unless you're melting chocolate along with a significant amount of water or another ingredient like butter or cream, just a few drops of water (like what might be in a wet bowl) can make the chocolate seize into an unworkable mass, so be sure that all the tools that come in contact with the chocolate are bone-dry before you start, and don't cover the melting chocolate (condensation from the lid might drip into it).
Store chocolate in a cool, dry place away from strong-smelling foods.