Do macs have user accounts to allow multi users?

I'm getting our first iMac. However I'm worried about whats going to happen when me and my wife start using it and it starts syncing with both our phones and some cloud thing we both now have since getting iphone 4's.... We have different apple id's different iPhones with different contacts, I don't want hers she doesn't want mine. same can be said for calendars and music and photos and the works. maybe we're dysfunctional but I can't hack her thousand of photos and schedule popping up on my phone! But I want us to be able to share a home pc pretty much like we do now using different user accounts or some such.

I'm not especially computer literate but possibly less dense than this post sounds. Just a little concerned about apple autonomy losing photos or doubling them. Would two seperate user accounts fix this?? What about our family photos that we both want to be able to access without storing them twice?

There must be other people in houses with multiple apple devices used by very different people who don't want to share everything?