Hitbox
A hitbox is an invisible box (or more often a series of boxes) which define the rough shape of a model for purposes of damage-based collision detection. A typical model within a game is much too high-poly to perform real-time hit calculations on, so hitboxes are used instead.
A hitbox is different from a bounding box in that it is more complex and closer to the model's visible shape. The bounding box is used for movement-based collision detection, and is usually literally a single box.
Creating Hitboxes
There are a couple ways of creating hitboxes. You can create them manually from scratch through the .qc commands or allow 
HLMV to auto-generate a set for you and then modify them from there. If your QC file is getting cluttered, the best way to keep your QC files organized is to create a hitbox.qci file which contains your hitbox data. You can link to it from your model's primary QC file with $include hitbox.qci.
Console Commands
- sv_showhitboxes <integer>
- Displays the server-side hitboxes of the specified player index, bypassing any
net_fakelag. Generates a lot of traffic, so should only be used on local servers. - This has been removed from Valve's games due to the confusion it causes with regard to lag compensation, but is still in the mod SDK.
See also