Is it possible to have a mod that can receive information from a server plugin. Like by using packets. If so could I see an example or explanation.

I assume you mean a bukkit plugin, in which case, I doubt it. As far as I know, the plugin would have to be made specifically to interact with the mod using some intermediary like chat (or, as pointed out by Ernio, an api), for instance. I doubt this is what you want, though.

What type of communication are you talking about?

If the server is running mod and plugin you can obviously exchange data between - in most cases by using reflection.

If you want client to be able to receive plugin's packet directly - you would have to change some stuff on plugin side, forge packets have id put into buffer, bukkit don't (it looks different).

 

Best way is Client Mod <-> Server Mod <-> Bukkit Plugin, safest probably. Marked bold can be done with as i said reflection or external API that both parties would use (if such exists).

I think if you write in two file and the another file reads it or simply setup tcp connection to localhost

If you have a server mod does everyone have to have the mod to join?

If you have a server mod does everyone have to have the mod to join?

No. Even forge would not be required. (theoretically)

http://www.minecraftforge.net/forum/index.php/topic,28953.0.html