From the installation window pop-up. Standard stuff, system requirement is worth looking.
Use iTunes to import CDs or download songs and videos from the iTunes Store. Make playlists perfect for any occasion. Burn playlists to CDs and play them on your home stereo. Sync your media with iPod or iPhone to enjoy it on the go, [...]

Apple recommends upgrading to OSX 10.5.3
Google blog talks about Address Book sync with Google Contacts. I wonder how many more years before we see iTunes and Blogger integration.
Here is a complete list of changes and fixes in 10.5.3.
General

Fixes a font issue that could result in Helvetica Narrow being used in applications instead of Helvetica.
Addresses [...]