What is a delegator?

Delegators are token holders who cannot, or do not want to run a validator themselves. Coin holders can delegate them to a validator and obtain a part of their revenue in exchange (for more detail on how revenue is distributed, see What is the incentive to stake? and What are validators commission? sections below).
Because they share revenue with their validators, delegators also share risks. Should a validator misbehave, each of their delegators will be partially slashed in proportion to their delegated stake. This is why delegators should perform due diligence on validators before delegating, as well as spreading their stake over multiple validators.