These terms are relevant to a discussion of Hammer.
A Map or World in Hammer represents the sum of all the resources in the game: all the code, all the textures, all the sounds, all the entities, and so on. When your players walk around in your game, it is a world that they’re standing in. Worlds also divide your game up into sections where different parts of the game take place.
In other games, these are sometimes referred to as maps, levels, scenes, or episodes. World editing is the primary focus of Hammer and each world is stored in a separate file on the hard drive.
Simply put, are anything a player interacts with that moves, lights up, shoots, growls, or goes into the player’s inventory is an entity. Hammer allows you to place any kind of entity that your games code or the engines code can create.
A Brush or Primitive is the basic unit of world geometry. A brush is made up of planes that define its faces, lines that define its edges, and vertices that define its corners. All brushes must be convex for reasons that will be discussed later. You can manipulate either an entire brush at once or any of the vertices, lines, or planes in the brush. Typical brushes are things such as cubes, pyramids, and cylinders.
A Prefab is like a primitive, but more complex. If you build a street lamp complete with textures and a light source that gives off just the right shade of light, you can select the lamp and it’s associated source (or any group of objects and brushes inside Hammer) and save it as a prefab. Thereafter, you can copy and paste that street lamp into your world without having to rebuild the whole object.
A Unit is the basis of measurement in Hammer and the Half Life engine. Units don’t equate directly to real world values. Instead, the game designers can select a scale of game units to real world measurements. As an example, in Half Life the following values are used: railings are 32 units tall, chest high crates are 48 units tall, and typical doorways are 128 units high. Also a player crouching would be 48 units, 64 at full stand, and 96 jumping.
When you Process or Compile your world, hammer creates a second version of the world with some information removed (parts only you need to know about), and other information added (parts only the game engine needs). Processing is a necessary step you’ll take in getting your world up and running in the game, and is often the first place where you will discover problems with your level.
The Texture is used like wallpaper, paint, or plaster to cover the raw plywood of your walls, floors, ceilings, doors and so forth. Without a texture, your brush will appear flat shaded in the game, almost always with unpleasant results. Just as you wouldn’t want a house with raw plywood walls, you always want to apply textures to the brushes in your level.