A share class defines the type of equity security held and the rights attached to it. Companies typically have multiple share classes outstanding at any one time: Common shares (held by founders and employees, last in line for proceeds) and multiple series of Preferred shares (held by investors, with progressively negotiated liquidation preferences, anti-dilution rights, and dividend entitlements added in each funding round).
Preferred series are usually labelled chronologically — Series A, Series B, Series C — reflecting the round in which they were issued. Each series may have different liquidation preferences (how much investors get back before common shareholders in a sale) and conversion terms. The stacking of share classes across multiple rounds is a key complexity in cap table modelling and exit waterfall analysis.