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.
All mods that dont add items/blocks to the game, or spawn items (cheat in anyway) will work
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
sometimes
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.
There are single player mods that enable this as a command. But most are for multiplayer if you are searching the plugin: WorldEdit is the most chosen
Yes, Minecraft mods are free. However, they can be harmful to your game and your computer and Mojang cannot do anything about it since they did not make the mod.
there are no mods in minecraft pe i hope there will be
Yes, most Minecraft Mods are updatable.
To enhance the game, and add new things into it.
No it might be that you have an out dated mod that crashes to game or that you do not have something like modloader (which you need for most mods). Also when downloading mods make sure they are for your current version of minecraft or it will defiantly crash.
Unplug your hard drive and put it in your computer and get mods for minecraft