iTunes is a complete management software for your iPhone. Though one thing which I never liked is when ever I connect my iPhone to my laptop, itunes start syncing music folder with my iPhone 3G s.
To stop this automatic syncing, you need to make a small change in the settings.
Follow the simple process :
Connect your iPhone to your laptop. Open iTunes and click on your iphone name. In the summery field , put a tick mark on the option which says :
Manually manage music and videos.
Find this screenshot to see how you can do it.
From now on, your itunes will not automatically sync music with your iPhone.