iTunes for Windows (source)
The Mac/Windows version of iTunes is a place where you do at least the following things:
- See your complete media collection (assuming you use no other media management software), and choose what music/movies/videos you want to play
- Playback any of the content you can see in iTunes
- Discover new content, using both catalogs and search queries
- Acquire content, by either downloading free content or purchasing paid content
- Sync content (including applications, et al.) with your portable Apple device
iPod touch (source: Apple)
In contrast, on iOS devices such as iPod touch, there are three distinct applications to do what all could be done in iTunes alone on a Mac/Windows machine.
- See: Use 'Music' or 'Videos' applications. You can't see your media collection in iTunes!
- Playback: Again, you can't playback locally stored content inside this "iTunes"!
- Discover: Use 'iTunes'
- Acquire: Use 'iTunes'
- Sync: iOS