The texture-browser is used for both the Map Editor and Model Editor.
The main differance between the two is that the textures for the Map Editor are loaded automatically based on each games configuration location of its game files.
For the Model Editor skin textures need to be extracted from the game file(s) and imported (most reliable way) or linked to for the texture-browser to use and display them.

There are many ways of getting into the texture-browser. The most common way for both editors is by clicking on its main menubar item Toolboxes then click on Texture Browser...

For the Map Editor another common way is from the polyhedron-view or face-view when you are looking at an object and want to select a texture for it.
See 'Texturing' for details on how to get to the texture-browser.

Across the top is the menu and toolbar, this is covered in more detail in the 'Menu and tool-icons' section.

Down the left of the form the first item you will see is the Searched Textures folder.This is where any textures will be placed when you use the Search tool, shown at the far right of the toolbar.

Next comes the Used Skin Textures folder. These are all the textures that are curently being used in the map or model as the case may be.

Below that is a list of texture groups. The ones shown here are from Quake III Arena, yours may differ depending on which game you are editing, and whether you have setup any additional texture groups of your own.

Most of the form is taken up with a preview of all the textures in the selected group.