STEP-1
First you need to authorize the computer to tranfer purchased apps from iPhone or iPod Touch. Run iTunes and Click on Store menu and then Authorize Computer.
STEP-2
This will prompt for the account information, enter the user name and password to authorize the computer.
This will start Access iTunes Store. wait for a while......
If computer is already authorized, you will get this message, Click OK button.
STEP-3
Now connect you iPhone or iPod Touch to computer and Click on File menu then Transfer Purchases from iPhone.
Simply Control+Click (Mac) or Right click (Windows) on iPhone or iPod Touch device icon and select Transfer Purchases.
This will start transferring purchased as well as free items from iPhone or iPod Touch to your Computer.
All purchased and free items from your iPhone or iPod Touch transfered to your iTunes/Computer, lets see where iTunes saves the apps.
On the left side Click on Applications menu under Libarary section and then Control+Click on application > Show in Finder (Mac) or Right Click on application > Show in Windows Explorer (Windows) This will open the folder where iTunes saves the apps or normally saves in \Music\iTunes\iTunes Media\Mobile Applications.
Using this guide you can transfer your apps to a new computer, from one computer to another, to iTunes, to iPod Touch, from PC to Mac and more ^^-OR-
