Yes, well, it's not exactly clear who has the patent on Tagging, but Google+ sure has taken a page out of other social net*works and given you this fea*ture straight up. To tag photos click on any photo to get its expanded full-size version and click the Add tag button.

Likewise, you can choose who can tag you. Go to Profile > Edit Profile> Photos tab. Her~ you can add or remove circles from the "People whose tags of you are automatically approved to link to your Pro*file" menu. By default, it's set to "Your Circles", which means anyone in your circles including Acquaintances.