Preferred stock is what investors typically buy in priced rounds. It carries economic rights (liquidation preference, dividend rights, anti-dilution) and control rights (board seats, protective provisions, consent rights on certain corporate actions). Each round typically issues a new series of preferred (Series A Preferred, Series B Preferred, etc.) with its own terms.
Preferred stockholders convert to common on IPO. In M&A, preferred holders elect either to receive their preference amount or to convert to common and share pro rata, depending on which yields more.