The Vocabulary

Before we jump in to creating your first room you'll need to learn some of the basic jargon of level design:

Brush

A brush is the name given to any 3D object with volume. Most brushes start in basic shapes - Cube, cylinder, wedge, and spike - and are grouped, carved, or in other ways edited to create more complex brushes like stairs, pillars, arches, etc. Brushes can also be non-solid types such as water, slime, or lava. The basic structure of a level is designed entirely of brushes.

 

Entity

An entity is pretty much everything else in the game that isn't a brush. Weapons, monsters, lights, player starts, etc.. are all entities. There's also a special sort of entity that combines with a brush in order to make it perform a function in the game: doors, teleporters, lifts etc. Just to make things confusing they're also called entities.

 

Texture

Textures are the 'paintings' on the brushes that make it look like brick, wood, or stone etc. Some textures are animated (the sky or water textures for example), and some give off light (such as some of the buttons.)

 

Face

A face is one side of a brush. Brushes will have 5 or more faces. A cube has 6 faces (Think of dice) Each face of a brush can have a different texture. A wedge would have 5 faces. Cylinders and Spikes can have many faces.

 

Clip Plane

Also called "clipping tool", this is one of the most useful tools for making brushes with irregular shapes. Essentially you use this to slice off pieces of a brush, or split a brush into two parts.

 

Vertices (and Vertex Manipulation)

A vertices are the corners of a brush. It's common to adjust the vertices of a brush in order to change it's shape. This is much easier than carving but can produce invalid brushes. (We'll look into that later)

Carving (or Subtraction)

Carving is another method level designers can use to create more complex brushes. It is the act of deleting the shape of one brush from another. It's somewhat akin to using a cookie cutter to shape brushes.
Care should be taken when carving, as mis-use can lead to map errors. See the Tips & Tricks section - Carving Carefully