Primary vs. Secondary Market
In the primary market, issuers sell new securities to investors and receive the proceeds; in the secondary market, investors trade existing securities among themselves and the issuer receives nothing.
An IPO is the first primary-market sale of a company’s stock to the public; later primary sales are follow-on (additional public) offerings. Once issued, shares change hands in the secondary market — on exchanges, over the counter, in the third market, or institution-to-institution in the fourth market, often through dark pools.