Yes, some may affect the game and its performance. If you have a slow computer, I don't recommend that you download too many mods.
You can download minecraft plugins from planet minecraft minecraft forums bukkit (for multiplayer) mods for minecraft.com mods for minecraft.net
All mods that dont add items/blocks to the game, or spawn items (cheat in anyway) will work
Survival and creative single and multiplayer
Getting onto Minecraft Multiplayer is very simple. All you need to do is when you get onto Minecraft you click "Multiplayer" instead of "Singleplayer", "Mods and Texturepacks" or"Options" and type in the Multiplayer IP (which you can get from the Internet or a freind) and click "Connect".
Rei's Minimap
There are definitely mods for that, like moreores, for Bukkit.
sometimes
Yes, they are.
Unfortunately skyrim itself is not multiplayer but there are mods out there attempting to make it that way
you can get minecraft mods and textures off the minecraft mods website.
Don't do that! You could get banned from the server as most servers dislike mods. Just be on the safe side and only use it in single-player
No, it is confirmed that there will be no Minecraft mods for Xbox. It was announced at PAX East that there would be no mods for Minecraft.