It depends on the mods-
Some mods are client-side only, and can be used without others needing them.
An example of this would be minimap mods such as Zan's or Rei's, UI mods such as NEI/TMI, or shader mods.
These mods don't add any new blocks or anything that your friend will be able to see-
They simply change things on your client.
Generally, if a mod adds new blocks or entities (such as tools or monsters) then it will only work if both you and your friend have it installed.
Other than that, a lot of mods will still work without your friend needing to install them too.
gaming.stackexchange.com/users/67962/Viprus
No, you can only play multiplayer if you have the same version of Minecraft and the same version of mods.
It is however fairly simple to install something like Feed the Beast and have multiple copies of Minecraft installed at the same time, this way your friend can have mods to play multiplayer and when he's playing alone he can play vanilla Minecraft without having problems with his save files.
As @dlras2 mentioned, it can work if you only have mods that are client side, like mini maps.
Basically anything that changes crafting or adds new blocks/items will not work without having the other player also having that mod (because all that is sent is "Hey, make this item" and the unmodded client will then go
"I HAVE NO IDEA WHAT TO DO
AND I AM PANICKING" and then probably crash)
gaming.stackexchange.com/users/244/Elva
It depends on whether they are client or server side mods.
Something like NEI ( Not enough items ) or a minimap is a client side mod.
These need to be installed only on the client.
They are things that add UI (User Interface) Elements.
Anything that adds items or mobs is a server side mod.
These need to be installed on both the server and the client.
So the short answer is maybe.
If you do not know which they are, please ask.
Hope this helps.
gaming.stackexchange.com/users/67961/jacoman891
It depends on what type of mod you are talking about, and whether you're are talking about LAN or actual servers.
If you're talking about LAN , then any mods that do not physically add anything into the game, such as TooManyItems , a minimap mod, or the shader mod , you should not encounter any problems.
But if it's a mod that adds new items, like RedPower , or BuildCraft , then you will start to encounter problems with LAN .
If you're talking about an actual server, then any mods that do not add anything into the game, like I said above, ShaderMod , TooManyItems , and others, you will not encounter any problems.
Yet, if they're mods that add items and blocks into the game, like BuildCraft or RedPower , as long as your minecraft versions are the same you should not encounter any errors when trying to connect, yet, if you attempt to get one of the items in the mod out of a Creative inventory, or another inventory, you will be kicked from the server.
If you're talking about putting mods on the server, all players will have to have that mod client-side to connect.
Long story short, If you're installing a client-side mod that doesn't add any new blocks, items, or entities, you'll be fine, If you get a mod that does add items, blocks, or entities, you will start to encounter some problems with LAN but you won't see as many on an acctual server.
If you are installing a server mod, all players will have to have that mod client-side to be able to connect.
gaming.stackexchange.com/users/68023/Jojodmo
Here's an answer from a more technical standpoint with examples: A minimap plugin might work fine just on the client, without a piece on the server.
This would be a minimap that works fine without a "server half".
Similarly, a plugin that shows you how many hearts of HP probably won't need to be on the server, because the normal client already knows all that's needed for it to work.
Some plugins, such as RedPower2, or AE set a "client must have" flag (in forge).
This means that if the server has the mod running, the client must have it to play on the server.
If the client didn't have it, the client would probably crash because there are blocks it doesn't understand.
(slight over-simplification)
On the other hand, if a client has AE installed, it can still connect to a "normal" server, because the plugin on the client is smart enough to "turn itself off" if the server doesn't have AE.
In theory you could also have the server turn off AE if an incompatible client connected, but this would be silly, since it means that anyone could disable that mod for everyone just by connecting with a "bad" client.
Some plugins only run on the server, and the client doesn't care.
These are usually plugins dealing with server permissions, message of the day, and plugins that do things when you make special signs.
A few plugins will cause your client to crash if the plugin is only on the client.
These are plugins that fundamentally change Minecraft so much that the server can't even talk to the client, or because the plugin changes vanilla blocks.
Usually:
*) If a plugin is for server admin, the client doesn't need it.
*) If a plugin adds blocks, modded clients can connect to a vanilla server, but vanilla clients can NOT connect to a modded server.
*) If the plugin is a small "player helper" such as a map, it will often work, no matter who has it.
*) Those last three points are guidelines, and not solid rules.
gaming.stackexchange.com/users/27599/Patrick M
I'm wondering if I could play Minecraft with my friend. A modded Minecraft, only myself with mods and him without mods.  Is it possible?
Can my friend and I play minecraft together with different mods?
gaming.stackexchange.com
<minecraft-java-edition><minecraft-mods>
gaming.stackexchange.com/questions/152887
gaming.stackexchange.com/users/67949/Asking12345678910
Some mods are client-side only, and can be used without others needing them. With other, you can only play multiplayer if you have the same version of Minecraft and the same version of mods.   It is possible for two players to play Minecraft, using different mods, as long as those mods are client side only.
The community states that client side mods don't have to downloaded by both players.