there are traces left in bitcoin network that may allow people or organization to link transactions and addresses to people.
for example, if you are in a exchange and send your docs to buy and sell bitcoin.
and your IP is revealed to the peers when you send a transaction if you don't mask your IP
As you can disguise your IP?
Yes you can "disguise your IP" by using VPN, some SPV wallets like MultiBit HD is working on Tor integration so that will honestly will fix most of the theoretical spying. However Tor is not 100% faultproof so taking basic precautions is still required or highly suggested.
Its only very hard to tell and prove who own an address without some user doing an action that reveal himself.