Once your iPhone is cabled up to the computer's USB port, click its icon in the iTunes Source list. The middle part of the iTunes window now reveals eight file-folder tabs, representing the eight categories of stuff you can sync to your iPhone.
Here's what each one tells you:
Summary. This screen gives basic stats on your iPhone, like its serial number, capacity, and phone number. Buttons in the middle let you check for iPhone software updates or restore it to its out-of-the-box state. Checkboxes at the bottom of the screen let you set up manual syncing, as described above.
Info. The settings here control the syncing of your contacts, calendars, email account settings, and bookmarks.
Ringtones. Any ringtones that you've bought from the iTunes store or made yourself (Chapter 10) appear here, so that you can specify which ones you want synced to the iPhone.
Music. You can opt to sync all your songs, music videos, and playlists here—or, if your collection is more than the iPhone can store, just some of them.
Photos. Here, you can get iPhone-friendly versions of your pictures copied over from a folder on your hard drive—or from a photo-management program like Photoshop Elements, Photoshop Album, or iPhoto.
Podcasts. This screen lets you sync all—or just selected—podcasts. You can even opt to get only the episodes you haven't heard yet.
Video. You can choose both movies and TV shows from the iTunes Store for syncing here, along with other compatible video files in your library.
Applications. Those useful and not-useful-but-totally-fun-anyway little programs from the iPhone App Store get synced up here. You can choose to sync 'em all or just selected ones.
At the bottom of the screen, iTunes displays a colorful map that shows you the amount and types of files: Audio, Video, Photos, and Other (for your personal data). More importantly, it also shows you how much room you have left, so you won't get overzealous trying to load the thing up.
The following pages cover each of these tabs, in sequence, and detail how to sync each kind of iPhone-friendly material.
These discussions assumes that you've (a) connected your iPhone to the computer with its USB cable, and (b) clicked the iPhone's icon in the Source list at the left side of the iTunes window.