The best way to keep everything separate (ie. music, contacts, email, pictures, etc.) is to have separate user accounts on your computer and separate Apple IDs.
I'm including some general information below for you regarding Apple ID's and iCloud.
An Apple ID is a user name you use for everything you do with Apple. Creating an account for an Apple service, such as the iTunes Store or the App Store, creates an Apple ID. Apple ID allows you to access other Apple services. You don't have to create a new account for each service—just use your Apple ID.
What is iCloud?
iCloud stores your music, photos, apps, contacts, calendars, and more, and wirelessly pushes them to your iOS devices and your computers. iCloud is integrated with your apps, so everything happens automatically.
For example, if you buy a song in iTunes on a Mac or Windows computer, it appears on your iPod touch and iPad. When you snap a photo on your iPhone it’s automatically sent to your iPad. You can start creating a Pages document on your iPhone, and put the finishing touches on it from your iPad. There’s no need to dock your iOS device or sync it to your computer.
Select Info tab to synchronize contacts, calendars, etc.
I hope this information helps ....
Have a great day!