A mod (also known as a plug-in or plugin) is a modification to the game code. If you are looking for information on the player level, then please see the Mod page.
There are two types of mods - server-side and client-side. At uRPS we frown upon client-side mods of any more complexity than texture packs and shaders. If we discover players using them, particularly for cheating, such as flying, or gaining blocks etc. then a ban is likely to follow. Note that this forms part of the Rules.
The server contains many mods, mostly for security and stability, and it would be pointless to list them all here (although for the curious, there is the command /plugins). However here is a list of the main ones (normally used to give players additional features in the game - find here a complete list of commands):