Very simple:

A NIP is a formal process used by the Nostr community to propose ideas, suggest changes, and make improvements to Nostr. Anyone can propose a NIP and get feedback from others in the community; they can showcase code and examples or provide a way to test their idea.

A NIP (Nostr Improvement Protocol) is a very informal standard for conveying ideas around how Nostr events and messages should be structured at a technical level to achieve a desired result.

Nostr clients individually decide which features of NIPs they will support, and those which support the same NIP are considered or intended to be compatible.