A zap is a direct payment from one user to a lightning wallet.
A boost is direct payment from one one user to a split block.
A boost will have as many zaps as entries in the split.
Zaps are to people, boosts are to a collective enterprise with an open contract for distributing zaps.

Zap's mostly use lnurl, Boosts mostly use keysend.
Boosts have significantly more metadata about the content being boosted.