iTunes gives you two ways to quickly see what digital goodies are loaded on your iPod. That info's important if you sync your iPod manually and can't remember if you copied a particular album or video over to the iPod. It's also helpful if you want to load a bunch of new movies, or other space-hogging files, and aren't sure how much room you've got left on your iPod.
To see what's on your iPod, click the flippy triangle next to the iPod's icon in the iTunes Source list. Then click any category—Music, Movies, and so on—to have iTunes display all the stuff under the selected category.
Click the iPod's icon and then click the Summary tab in the main window to see a color graphic displaying how much of your iPod is filled with audio, video, photos, applications, and so on. If you want more information than say, "12.5 gigabytes," click the colored bar itself to see the statistics change to the number of items in each category (like 440 photos or 3396 songs), or how many hours (even days) worth of each type of media your iPod is holding for you.