Using the Console
The ZDoom console can be accessed by pressing the tilde key (~) by default. When pressed, the console will appear from the top of the screen and extend downwards to fill half the screen during gameplay or the entire screen otherwise. If in a game, this will also cause the gameplay to halt while the console is shown. Commands can be typed directly at the prompt in the console, allowing the user to modify a variety of game settings, activate cheats or debugging features, and otherwise take direct control over the game engine.
When using the console, there are a few hotkeys that have a special function. You can scroll the console forward and back using the Page Up/Page Down keys, or by scrolling the mouse wheel. When the console is scrolled up, the prompt will remain visible, but there will be a row of uppercase V's above it indicating that some of the console log has been hidden.
Pressing the up and down arrow keys will cycle through a list of recently-typed commands. In addition, you can type a partial command or CVAR, and then press the TAB key to use the auto-complete feature. This will attempt to complete the partial command with the best match found. You can continue pressing TAB to cycle through all matching CVARs/commands until you find the one you want.
For example, typing vid_draw and then pressing TAB will auto-complete the CVAR and display vid_drawfps, with the cursor positioned after the 's'. You can then press enter to display the CVAR's current setting, or enter a new value to change the setting of the CVAR. You can find more information about using CVAR's here.
You can close the console and return to the game by pressing the tilde key (~) while the console is open. Certain commands will automatically close the console once they are executed.
Console variables, or CVARs, are variables that may be changed by the user that control everything from the game type to the number of decals to display on the walls. They are typically divided into a number of categories and identified based on their prefix. For example, vid_drawfps is part of the Video category, while cl_capfps is part of the of the Client category of variables. Not all CVARs will fit this mold, however.
Console commands, or CCMDs, are commands that can be executed from the console. Unlike CVARs, they are not categorized by name. However, for the purposes of documentation, the wiki does divide them into several categories based on their area of use.