Syncing contacts - does it work both ways?

[sushidushi]

I have made a few changes to my contacts' details over the last couple of days, both online and on my phone (X10a).  If I sync the contacts, will the online stuff over-ride the changes I've made in the phone, or will the phone take priority?  I'd like it to give me a choice of which changes to keep.  If it uses the details i have most recently changed, that would be ideal, but I don't really want to try it until I know what will happen...

Any advice would be gratefully received.  Thank you!

[sushidushi]

I hope people don't mind me bouncing this to the top.  I really don't understand sychronising and would like to try to find out how I can ensure that changes I make to the phone are reflected on the computer, and vice versa.  Not just with contacts, but also photos and music and so on.

Also, related to that, I think, when it comes to updating the phone, how do I ensure that I don't lose information?  I think I have to back it up, but is that on to the computer, the memory card, or neither of the above?

[#3]

Yes, It works both ways. When there is change on both side it keeps the copy of both. So no loss of information. We can also merge duplicates to one contact by logging in to SE website.