Newb questions

Just ordered a SD2 and have been scrambling to learn, learn, learn. I assume a raft is the squigglie base laid down before the part? Is this drawn in, or part of the software? What is the need, I would figure you would want the first layer of the part on a flat surface. Also I would like to learn more about using glass as a platform, but don't see any "basic" discussions on it. Thanks