Brushes and patches in NetRadiant can be textured; these textures are projected onto each face, and if the brush moves, the texture will "slide" across the surface of the brush. All six sides of every brush can be textured and mapped independently with the surface inspector.
Textures vs Shaders
A shader is a dynamic texture, generated in real-time with GLSL. Many of the images listed are not actually textures but shaders instead; shaders make dynamic textures such as lava, water, glass, and skydomes possible. Note that shaders are not used in the camera view; they will instead use the shaders' default texture.
The texture browser (press T if you have switched the layout) shows the currently available texture folders on the left and the textures or shaders within that folder on the right. Double-click on a folder on the left to show its textures, and click on textures to apply them to all selected objects. Applying textures to objects will not change the texture mapping; if you switch between differently-sized textures on an object that has been previously fitted, you will need to fit the texture again with the new size.
If a texture or shader is currently being used in the map, it will have a green outline in the texture list; if it is a shader, it will have a white outline; and if it's currently being used and it's a shader, it will show the green outline outside of the white outline.
The surface inspector (hidden by default; press S to toggle the window) handles the mapping of the texture; it can also fit and align textures to brushes, patches, and curves.
There are three ways to move textures; translation ("sliding" textures), scaling (stretching textures), and rotating. All three can be manipulated independently with the number entries on the left side. (The "step" number entries on the right adjust the offsets applied when you click the "up" and "down" arrows on the left-side number entries.)