Cloud is dedicated for congruency across your iOS devices. Depending on what you have enabled it will sync your pictures, videos, messages, calendar dates, notes, mail, reminders, and bookmarks across your iOS devices. If you install the cloud control panel you can also have some of that media on your computer as well. This way, when everything works right, when you walk out of the house, you can continue the conversation you were having on your iPad on your iPhone or iPod touch.
Cloud also provides backup and restore services. By default it should back your data up when your device is plugged in, locked, and connected to wifi. This way if anything should happen you just have to connect to WiFi and tap "restore from cloud" and all your data will be put back on your iPad exactly where it was.
For music and such check out iTunes match as described above.
For documents and other data that you want to have on your PC and iPad I would suggest Dropbox. It has saved me many a time when I've had a presentation or something and either lost a thumb drive or had a corrupted file. All I had to do was hook my iPhone (you can use an iPad too) up to the projector and open my presentation through Dropbox. It works great for music and video as well. I keep a lot of stuff on there that I don't want to be without but that I don't want hogging space on my iPad. It's free so check it out here
http://db.tt/MjWA6Li0
Also, I realize I sound like an advertisement for both Cloud and Dropbox and I'm sorry, it's just a little late here.