Investors often ask for rights to be paid back first in exchange for their investment. The way these different rights are handled is by creating different classes of stock. (These are also sometimes called classes of shares, though that term has another meaning in the context of mutual funds.)
The exact number of classes of stock and the differences between them can vary company to company, and, in a startup, these can vary at each round of funding.
confusion Another term you’re likely to hear is founders’ stock, which is (usually) common stock allocated at a company’s formation, but otherwise doesn’t have any different rights from other common stock.*