NPC= "Non-Player Character"
Which means exactly like how it sounds. You can't control the character, but they help you in the game.
The not-as-important NPC's
They give advice or items, and they do in-game trades. Those types of NPC's are the characters like Youngsters, Veterans, ones like those.
More Important NPC's
The Pokemon Professor, your rival, Gym Leaders... These are important.
They are: Professor Juniper, Bianca, Cheren, N, and Nate/Rosa.
A more complete list would be here, if you wanna see all of them