So I installed Fallout again to try this out, and it seems that when you save your game, mods that are not present at the time of save (even if they were active at some point before) are erased. So reactivating mods later wont work.
This entirely depends on what the mod changes.
If you have a mod that changes things that only the client cares about (e.g., adds realistic shadow shaders) then the client mod will work fine in multiplayer. (These will not have any effect installed on the server.)
- A UI mod will only change clients. Installing it on the server will have no effect.
If it changes something that only the server is responsible for managing (e.g., changing the world generator algorithm) then the client doesn't have to know about the mod at all and it can be server-only. (These will not have any effect installed on the client when playing multiplayer.)
If it changes the world in a way that client and server must both understand (i.e., new items/blocks), then it has to be on the server and the client. (What happens if there is a mod mismatch depends on the mod.)
- In your specific example, no, adding new blocks to your client will not let you use them on a multiplayer server. You might be able to join, or you might not. You might be able to join, but get kicked as soon as you try to use a custom block. It entirely depends on what code you change. The possible interactions are as many as there are lines of code in Minecraft that can be changed.
- Adding a new-block mod to a server will make it incompatible with clients. This might manifest as disconnects, crashes, or client-side "ghosts". The client must have a client-side mod that matches the server-side mod.
- A permissions mod is server-side only, but a UI mod is client-side only. To make a UI mod able to change server-side permissions, you have to have a client mod and a server mod that know how to interact with each other.
These things entirely depend on how the mod is coded. Without knowing what your mod changes, we can't even guess. More technically, where server mods start and client mods begin are a matter of what specific Java functions they change and how those functions operate as part of Minecraft's server-client architecture.
Best Answer
Clientmod: A clientmod is a simply modification which mostly just affect the look of your game or the gameplay (without any unfair advantages!). So this could be a texturemod or just some different soundfiles or something.
Servermod: A servermod is needed to manipulate the game in a significant way. So there are servers which kick a player who didn't use a specific weapon or where you can't use your own classes your created (ref. CoD). Instead the servermodded lobby offers you some predesigned classes to achieve a specific kind of gameplay on this server. Servermods can also be little modifications like changed soundfiles (UT-Sounds in CS for example). Besides that there are servermods/tools which make it easier to administrate the gameserver, but I won't call this a mod, it's more an admintool.
Keep in mind that: