And I would imagine which content. We won't know for sure until this Fall.
Your iPod Classic would still have to sync manually (with a cable), because the service is designed for devices with wireless communications.
As far as media, Apple is launching with Music, Photos, Documents (only Apple apps at first, third party coming later), Books, Apps, Device settings, App data (and more system related things/settings), Contacts, Calendar, and Mail.
No video, yet. It'll come with time.
Like I said, you will have choices on what to 'automatically sync.'
Basically, it's a replacement for MobileMe (minus some parts, plus others) done better.
It's free with 5GB of space, but your
purchased music, apps and books, plus your photos, don't count against that storage.
My additional guess is that they will charge for additional space. At least I hope so.
There is only one 'add-on' pay package so far called 'iTunes Match.' For $25 a year they will store up to 25,000 songs for you and let you download any of them on demand. Even if you have a lower quality MP3, they will give you a higher quality music file. And you don't have to upload your files. They scan iTunes and 'match' your music.
My wife and I are Really excited about this, but hope that they have a bigger package. We have closer to 40,000 songs in our collection.
Anyway, you don't even have to use the service. And like I said above, we don't know the final details.
For more:
http://www.apple.com/icloud