If somebody plugs there Iphone into my computer, where does his information go?

Photos might be copied to the image capture application or to iPhoto on a Mac. If you mean that you connected someone else's phone to iTunes on your computer and then hit apply or sync, any info from a previous sync will be erased and not copied to your computer. Usually a backup is made as part of the sync process but that backup will not have all the data that was on the phone. The best way to get the original stuff back is to sync the phone with the computer that the phones owner synced with previously.