Find the world in your 'saves' folder (which is in your minecraft folder) and copy it to another location, like Documents or Desktop.
If they've all been removed because you completely removed Minecraft from your computer and deleted the minecraft folder, then there's no way to get them back unless you have a backup of the minecraft folder saved somewhere else. If you have a backup just copy the texturepacks folder across. Sorry though, there's no way I know of to get your servers back.
Well I dont think you can you would have to search on google for a download sorry.
It means, that your account is hacked. delete your .minecraft folder (backup save files) and change your password on minecraft.net. Redownload minecraft and it´ll work.
You may have corrupted the world by using mods or other unlisenced programs to edit the game. It is best to revert to a backup that you have saved before.
Yes there is such thing as the Ether World in Minecraft.
Get a ghast to shoot it's fireball into the nether portal and if the portal is gone and you have no obsidian you are stuck and must make a new world, thats why you should always make a backup
Yes it does, i have it myself. Be sure to backup minecraft and delete META-INF Or even watch a tutorial. Have fun!
An underwater world seed for Minecraft is used to open an underwater world to play the game in.
Sorry it's probaly not possible ):
Yes, Minecraft is global.
No
Place the world in your minecraft bin's saves folder.