Syncing my iphone will change more than 25% of my contacts?

I'm trying to since my iphone 4 onto my computer and it keeps telling me that if I sync it, more than 25% of my contacts will be changed on this computer. What does this mean? Will anything be erased and how can I prevent it from being erased? Someone help me out here please? :/

Back up 1st.
then sync.
if it erases anything just restore off your back up.