The Home Sharing feature of iTunes allows you to share specific types of iTunes content between computers on your home network. It works by allowing the iTunes software on each computer to sense the other computers on the network and read their iTunes libraries. Apple limits Home Sharing to five computers, but you can always deactivate an unused computer to add a new one, and then switch back later if you no longer use the new computer.
From the "Advanced" menu bar within iTunes, select "Turn on Home Sharing" to activate sharing on the computer that holds the iTunes files you wish to share with your other computers. After you set up Home Sharing on this computer, turn on Home Sharing on your other computers and sign in with the same Apple ID you used on the first computer. On each of the computers, the Home Sharing icon will appear in the left side menu, and you will see the content of the original computer.
The actual media content is not stored directly within iTunes, so the content is not instantly shared, only your library information. This means that to play a song or movie from one computer to another, you must stream it across the network or import it into the second computer's library. If you choose the former method, you can only play the content on the second computer if the computer that holds the file is on and signed in to iTunes on the network. In the latter case, after the file is imported you can turn off the first computer and play it on the second computer on any network, as long as that computer is still authorized.
ITunes Home Sharing can only share media files that iTunes has the rights to share. This means that content you did not purchase directly from the iTunes store may not show in your Home Sharing section. Rental movies from iTunes also cannot be shared through this feature. Home Sharing will share between different operating systems, including various versions of Windows and Mac OS X. Each computer must have a version of iTunes that supports Home Sharing.
If you have Home Sharing set up on your computers but the Home Sharing icon and files don't appear on the left menu, you may have security settings that prevent your computers from sharing files across the network. Configure your network as a "Home" rather than "Public" network, and you may need to alter each computer's settings to allow file sharing. ITunes also has a setting in the "Advanced" menu to allow others to see your files, so make sure this setting is active.