An equity incentive plan (also called a stock plan or stock option plan) governs compensatory equity awards to a startup's employees and other service providers. In general, startups will grant equity awards via the equity incentive plan.
The stock reserved for issuance under the equity incentive plan is often referred to as the option pool, though of course these shares can be used for different types of equity awards, including stock options, restricted stock, stock appreciation rights, restricted stock units, and more.
Equity grants made through the equity incentive plan will usually be exempt from securities registration requirements under Rule 701 (at least federally).