The question has been closed for the following reason "SalvorHardin won 1.00 µBTC for the best answer." by mandette 19 Mar '13, 00:30
Are you using the latest 0.8.0 or 0.8.1 version? Using the new database structure makes the new client syncing extremely fast, at least comparing to prior versions. A less likely reason is that your hard disk drive is extremely old or slow or fragmented.
Unless you are connecting to the internet all the time, each time you start the client, it will try to sync with the netowrk and download the latest blocks. There are around 150 blocks found per day, so there will be some syncing needed every day. But with the latest client, it's really much faster than before. There are some "light" client as mentioned below. They only download the data pertaining to your wallet, and not the whole blockchain. But it does require that you connect with a central server, making it more vulnerable to attacks like DDoS.
It's out of sync when it is not always connected. The syncing happens because it's trying to catch up.
There are wallets that can stop or cut down the need for sync. These are known as Simplified Payment Verification, from clients to websites.
Here's examples of where to look to if interested: Paytunia, Multibit
answered 18 Mar '13, 16:05 flug 704●7