It pretty much lets everyone have their own unique apple ID, apps, playlists, and setups.
The only real connections, are that they share a single billing system (ie, whatever credit card that is set on the main person's account). But they can each add their own itunes card funds to have separate funds if needed/wanted. So anything bought will first come from available funds on that person's account, then if not available will bill the card on the main account.
Also, any apps (etc) purchases on any of the family accounts, will be available for use by anyone else on the family account.
You can also set it up, where minors have to request permission from an adult on the account in order to purchase a new app (or download a free one). I find this very useful with my younger son, who would download anything he could.
Another benefit, is that all the families iphones/ipads will show up on the "find my iphone" app. Very, very useful for when the kids are away from the house, and you want to keep tabs on them. Each phone does have the option to not share it's location with the other family members, but my kids know better than to turn that off.
All in all, family sharing is a very worthwhile addition.
As for how it works on the imac, I'm not certain, as I'm not really a mac user. Does the mac allow seperate logins like a windows pc does? If so, then it should be no problem.