Non-player Characters, NPCs, are characters controlled by the game software.

These NPCs give you rewards for gathering items for them.
Contacts are mission givers. They either work for an organization or are independent.
Iconic Characters
You will meet a number of characters from the Matrix movies.
Mission Characters
Many mission objectives involve speaking to, giving/taking items to/from, or escorting NPCs (one generally doesn't consider named MOBs that are merely kill targets as NPCs).