Nonplayer Character

Characters inhabiting the game world who are not controlled by players are referred to as nonplayer characters (NPCs they are controlled by the GM.
Nonplayer characters can be allies or enemies, guides, mentors, antagonists, or trading partners. They can play a large part in the story, or meet your player character a single time.
NPCs have the potential to include just as much detail and nuance as the characters you create, from Alignment, to Ability Scores, to backgrounds, personality traits, ideals, bonds, and flaws.

Interacting with NPCs

Most interactions you have with nonplayer characters will be social in nature. Based on circumstance and some elements of random chance at the discretion of the GM, all NPCs will hold an attitude toward you -- friendly, neutral, or hostile. This attitude can change based on your interactions with the NPC and changing outside circumstances.
When you want something from an NPC -- whether a purchase, or a solicitation of assistance, or some other goal -- you will engage in negotiation. When an NPC wants something from you, they can engage you in negotiation, as well.
When negotiations break down, or when an NPC's hostility becomes overwhelming, you may end up in combat.
On the other end of the spectrum, you can form close bonds with NPCs that turn them into allies and companions who will aid you in conflict (see below).

NPCs in Conflict

All NPCs have a statblock that lays out their game statistics and combat abilities, just like any creature. They also have an inventory that lists their possessions.
When an NPC interacts with the game world, they use their ability scores and proficiencies just like you.
When faced with a threat, they can use their abilities and their possessions to help them win the conflict., including during combat.
If you want an NPC's possession, you can attempt to negotiate for it, fight them for it, or even steal it with a successful Dexterity (Sleight of Hand) check.

NPCs as Companions

You may grow close enough with an NPC to develop a trusting bond of companionship, or they may follow you as a loyal retainer. In these situations, the GM may grant you access to their full inventory and statblock, and allow you to control the NPC in combat. Usually, the GM continues to control a companion during social interaction.
These relationships may change, however, and at any time the GM may take full control of the NPC again as circumstances and story demand.