In Twitter or Facebook you can tag people, by using the @ syntax to refer to them.
What that means is:
Other people can find the person. It's the equivalent of linking in these systems.
The person being talked about will see it.
The first effect is good, always, the second, often is not.
Tagging is loud
When you tag someone, you're saying "Look at what I said about you." Then the question is, did the person you're tagging enjoy the experience?
I like to know, of course, when I'm being talked about, for my work, even being called out for something the author doesn't like, assuming they say so in a respectful way. But that often isn't why people use tagging.
The other day, a Twitter user said he was unfollowing me, sadly, because blah blah blah. He tagged me in the tweet. Did I need to see this? No. In fact, I have a motto that covers this situation: Don't slam the door on the way out.
Leave quietly, that's the classy thing to do. That way you're sure what you're doing is leaving, and not punishing, or asking for an apology, asking the person to change, or asking to be talked out of leaving. If you're doing any of those things, have the guts to say it directly. By not tagging the person you blame, you're making sure you're leaving for the reasons you say you are.
Another related motto from my longtime friend David Jacobs: No old tomatoes. Which means, no ultimatums. For the same reason as not slamming the door on the way out. If you're wanting to leave, just go. No drama. No negotiating. It's tough not to lay it out. But ultimatums, as tagging often is, are powerless. Imagine what it would feel like to have your bluff called, and then skip the bluff altogether.
Tagging as spam
Another reason people tag is to spam you. They put your tag in a message about their new video, or an article they wrote, or a post they want you to link to.
An example
The other day I mentioned Jay Rosen in a blog post, and linked to a piece he wrote. I didn't tag him, in my post, or in the follow up discussion on Twitter or Facebook. I'm not tagging him here. Will he see this? If he was meant to, in some karmic sense, he will. But I'm not going to force it. Let the network do its thing. It's usually pretty good at it.
Muting as the anti-tag
BTW, the new Mute command on Twitter is an antidote to unwanted tagging. People who tag too much should take note. If you tag someone gratuitously, now, you might get muted.