I know what a game mod is. But what is a server mod? Do clients need to install the server mod to get the features of that server mod?
Also, if I have a mod for a game installed on my computer, and I play the multiplayer version of that game, will I be able to enjoy that mod's features if the server doesn't have that mod installed?
From a development perspective, anything you would have on a usual server (world objects etc.) must be modded into the server mod while things such as rendering those world objects, must be included in the client mod. If the client mod has no need to have data from the server that it can't already access, then it doesn't need a server mod.
So a mod that adds flying cows throughout the game world, would need modification to both the server (to control, spawn, etc.) the cows, while the client would also need a mod to display the cows. However if the mod simply added the scoreboard to the top of your screen, you wouldn't need a server mod, because chances are the scoreboard is already being sent from the server anyway.
Server mods are just a sub classification of game mod. It's a modification to the way the server processes data. This could be anything from extra logging to a text file to changing the core mechanics of the game being played.
The client side requirements depend on what the mod does and the game the mod is applied to. Depending on the game, the server may have lots of control over the client or very little.
The more control the server has the less likely it is you'll need to install the mod on the client too. This would include the client getting resources from the server like new maps, textures and sprites or 3D models.
The less control the server has (i.e. clients do all the simulation and the server syncs it, there is no transfer of content from server to client), you'd likely need to install the mod on the clients as well.
Further, the above also depends on the extend of the modification. The more the mod changes the more likely you'll need to install it on the client too and vise versa.
Depends on how that game is made and what the mod change.
See Minecraft. I think I can arrange probably all mods on 2 distinct groups: UI mods and Mechanic mods.
UI Mods change one the appearence of the game. Like textures, chat mods and etc. Mechanic Mods and or change some behavior of the game. Like adding new drops and blocks, add a new monster, add quests or economy to the game.
Most of (if not all) the UI Mods change the game only for you. Other players don't know or see what or how you see the game. This kind of mod don't need to be on server side, only on client. For Mechanic Mods it a bit different. On singleplayer all the mods need to be installed on client, of course. On multiplayer, some changes on the behavior of the game don't need additions in the client, like altering the rate of droping some item, or changing a monster spawn time. But some other mods need changes in client. If you need to add a new monster you need to add it to the server. But need to add a new model of this monster in the client too.
Like Kikaimaru and I said, its depend on what is the game and how it was made.
