If I make a forge server with mods, do other people need the mod? [duplicate]

Clients need vast majority of mods the server has.

• The version of mod has to be same on server as in client, if they are on both sides
• Forge is only the loader of other mods

Rules for what mod has to be in client/server or both:

• Any mod that adds any block, item or entity; or mods that offer network communication : Both server & client
  • sever needs to know what you are holding and client needs to know it either, so it can show it for you. NEI/JEI needs to be also on server, if you want to have weather/time buttons working, etc.
• mods that add some server feature : typically only server
  • Typically functions similar to "bukkit plugins", for example ForgeEssentials, or Morpheus (modifies only the number of people need sleeping to get a day), or DiscordChat etc.
• mods that change the client's way of rendering, but don't add anything into game : typically only client
  • Optifine, minimaps, shaders, texture packs... (those can even stop server from functioning if you try to put them there)

Be aware that there are some mods that allow server-configuration for purely cosmetic/graphic mods, those have to go both on server and client, if you want to sync settings (respectively you want to force server settings to people playing on server). For example HardcoreDarkness mod is optional on server, but if it is there, it is a must for joining clients:

If you install this mod on a server all clients will have to have Hardcore Darkness installed. Clients will also be forced to use the config file you specify on your server.

Some mods have different version for server and client, for example JourneyMap or ForgeEssentials, then you should have the corresponding ones on both sides, however those are normally optional on both sides. Every mod should have such an info on their project site.

You should also consider to sync whole config folder to clients, because various settings can reject clients from joining the server. For example AE2 mod offers settings to disable items, then if you disabled item and join server that has it, it won't let you in.

If you have version 1.8+ and your server has only servers-only mods, even pure vanilla clients can join (you can take advantage of this!). For 1.7.10 and lower versions you are not able to join Forge server without Forge installed in client, even if there are no mods.

Also be sure that if you use classic minecraft launcher, to use standalone profiles , that are setupped with custom directory in profile editor. That is needed to avoid conflicts between versions, even with vanilla ones! Also more RAM is needed to load all parts of the game.