Data – the data or info category consists primarily of things like your contacts, calendar, bookmarks, notes, email accounts, etc. This information can either be syncd via iTunes or wirelessly via MobileMe or Microsoft Exchange (although Notes currently can only be syncd via iTunes).
Media – the media category consists of your music, movies, music videos, TV shows, podcasts, audiobooks, ringtones, iTunesU and now iBooks. This content can either be syncd or managed manually.
Photos – well this category is pretty self explanatory. Its your photo library and all of your photo albums. Your photos can either be in iPhoto or simply in a folders and subfolders.
Apps – last but certainly not least is your Apps that youve downloaded from the App Store.
Now that you know what the four categories are the content from these four categories can live on one, two, three or four different computers. In my house I sync my iPhone, iPad and iPod touch between two computers. I have a MacBook Pro that is my main everyday computer and we have an iMac that is our 'iTunes media server'. Of the four categories above I sync my Data, Photos and Apps with my MacBook Pro and I sync my media with my iMac. This works great for me because it means that I dont have to keep every song, movie, TV show, etc. that I own on my MacBook Pro.
How do I sync between more than one computer?
This is where most people get scared off. First off the categories cant be split if youre syncing. So photos have to be on one computer or the other, not both. Same with Apps, etc. The only thing you could get away with splitting is if you set your media to 'Manage Manually' then in theory you could drag content onto the device from more than one authorized computer. I prefer to use syncing, which brings me to the big scary message.
When I set up a new device I start with my MacBook Pro and in iTunes I UNCHECK 'Open iTunes when this iPhone is connected' and then I UNCHECK Ringtones, Music, Movies, TV shows, Podcasts, iTunesU, and Books. Basically all of the media category. I perform my initial sync and all is good. Then I unplug the device and move to the iMac and plug it in. In iTunes I uncheck Info, Apps and Photos and I enable all of the media categories. Now when I hit the sync button Im going to get a big dialog box warning me that this iPhone (or other device) is syncd with a different iTunes library and if I continue its going o erase the content. Thats what stops people dead in their tracks. YES! This is OK, all it means is that its going to erase the MEDIA content, not the other content from the other computer. It will put on the media you just enabled. Once this second sync is done your Apple device is now configured to sync between the two computers. You can go back and forth syncing between the two whenever you need to. So If I want to sync new notes , Apps or photos I sync with my MacBook Pro. If I want new movies, songs, or iBooks I sync with the iMac. Ive been doing this ever since the first iPhone came out in 2007 and most recently with the iPad. It works great!