All the information here is taken from Unreal Documentation.
Project
An Unreal Engine 4 Project holds all the contents of your game. It contains a number of folders on your disk, such as Blueprints and Materials. You can name and organize folders inside a Project however you wish.
Level
A Level is a gameplay area that you define. Levels contain everything a player can see and interact with, such as geometry, Pawns, and Actors.
Unreal Engine saves each level as a separate .umap file, which is why you will sometimes see them referred to as Maps.
World
A World is a container for all the Levels that make up your game. It handles the streaming of Levels and the spawning (creation) of dynamic Actors
Actor
An Actor is any object that can be placed into a level, such as a Camera, static mesh, or player start location. Actors support 3D transformations such as translation, rotation, and scaling. They can be created (spawned) and destroyed through gameplay code (C++ or Blueprints).
Casting
Casting is an action that takes an Actor of a specific class and tries to treat it as if it were of a different class. Casting can succeed or fail. If casting succeeds, you can then access class-specific functionality on the Actor you cast to.
Component
A Component is a piece of functionality that can be added to an Actor.
When you add a Component to an Actor, the Actor can use the functionality that the Component provides. For example:
- A Spot Light Component will make your Actor emit light like a spot light.
- A Rotating Movement Component will make your Actor spin around.
- An Audio Component will give your Actor the ability to play sounds.
Components must be attached to an Actor and can’t exist by themselves. Components created as sub-objects within an Actor are instanced, meaning each Actor instance of a particular class gets its own unique instances of the Components.
Pawn
Pawns are a subclass of Actor and serve as an in-game avatar or persona (for example, the characters in a game). Pawns can be controlled by a player or by the game’s AI, as non-player characters (NPCs).
When a Pawn is controlled by a human or AI player, it is considered to be Possessed. Conversely, when a Pawn is not controlled by a human or AI player, it is considered to be Unpossessed.
Character
A Character is a subclass of a Pawn Actor that is intended to be used as a player character. The Character subclass includes a collision setup, input bindings for bipedal movement, and additional code for player-controlled movement.
Player Controller
A Player Controller takes player input and translates it into interactions in the game. Every game has at least one Player Controller in it. A Player Controller often possesses a Pawn or Character as a representation of the player in a game.
The Player Controller is also the primary network interaction point for multiplayer games. During multiplayer play, the server has one instance of a Player Controller for every player in the game since it must be able to make network function calls to each player. Each client only has the Player Controller that corresponds to their player and can only use their Player Controller to communicate with the server.
AI Controller
Just as the Player Controller possesses a Pawn as a representation of the player in a game, an AI Controller possesses a Pawn to represent a non-player character (NPC) in a game. By default, Pawns and Characters will end up with a base AI Controller unless they are specifically possessed by a Player Controller or told not to create an AI Controller for themselves.
Player State
A Player State is the state of a participant in the game, such as a human player or a bot that is simulating a player. Non-player AI that exists as part of the game world doesn’t have a Player State.
Some examples of player information that the Player State can contain include:
- Name
- Current level
- Health
- Score
- Whether they are currently carrying the flag in a Capture the Flag game.
Game Mode
The Game Mode sets the rules of the game that is being played. These rules can include:
- How players join the game.
- Whether or not a game can be paused.
- Any game-specific behavior such as win conditions.
You can set the default Game Mode in the Project Settings and override it for different Levels. Regardless of how you choose to implement it, you can only have one Game Mode for each Level.
Game State
A Game State is a container that holds information you want replicated to every client in a game. In simpler terms, it is ‘The State of the Game’ for everyone connected.
Some examples of what the Game State can contain include:
- Information about the game score.
- Whether a match has started or not.
- How many AI characters to spawn based on the number of players in the world.
Brush
A Brush is an Actor that describes a 3D shape, such as a cube or a sphere. You can place brushes in a level to define level geometry (these are known as Binary Space Partition or BSP brushes). This is useful if you want to quickly block out a level, for example.
Volume
Volumes are bounded 3D spaces that have different uses based on the effects attached to them. For example:
- Blocking Volumes are invisible and used to prevent Actors from passing through them.
- Pain Causing Volumes cause damage over time to any Actor that overlaps them.
- Trigger Volumes are programmed to cause events when an Actor enters or exits them.