Delegation is the process by which ADA holders delegate their funds to a stake pool. It allows ADA holders that do not have the skills or desire to run a node to participate in the network and be rewarded in proportion to the amount of stake delegated.
Staking usually refers to to the global state of all staked resources (from pools and from delegations) and delegating usually is used when describing specific functionality related to how users perform delegation and how it’s accounted.