Slash commands provide a way to manage servers, change some settings, and request additional information (such as network quality information). Most commands are server-side, meaning the server processes them. A few are client-side, meaning they are processed by the client and never sent on the server.
Slash commands are sent via the chat mechanism, which can be initiated by pressing the n key (by default). After the command is typed, the Enter key will send the message.
Additionally, pressing o key will bring up a menu at the bottom of the screen that can be navigated through with the arrow keys. This provides a way to run various commands without manually typing them. The up and down arrow keys will choose the command to run, and the left and right keys can be used to select a player for commands that require that.
There commands are generally available to normal players, though some servers may restrict some of them to registered players.
There are various commands that are commonly available to server admins (or cops).
These are commands that generally only the server owner has access to as they allow configuration changes to the server.