Don't quite understand how syncing and the sort works...

[OkadaK]

Hi, I need to know exactly what happens when I hook my iPhone 4 (icloud) to a laptop. Also, I used a user account on my mothers Mac and it destroyed her contacts list, totally gone, and replaced it with what I had on my phone, and also did the same to her calander. I am afraid to connect the phone again, but I would love to know what I could do to isolate my stuff from hers on the same computer. I need to off load photos and music, and the sort. This all is very confusing to a guy who's been out of the loop since the early 2000's.