A tag is where someone links your name with online content. This could be a photo you appear in, or a video or link, or even a comment on a page. When your name is attached to it, you get a notification or message to say you've been tagged - and you can then view that content. It can appear on your profile, on your news feed - and can sometimes even turn up in search results.
However, while tagging can be great for sharing good stuff and staying connected, if you're tagged in something slightly dodgy or embarrassing, it can be difficult to remove - and lots of people may see it who you'd rather didn't. Like your employer, your family, complete strangers...
If in doubt, it's best to ask before you tag someone, and you can also set your privacy settings on social networks to prevent others tagging you, or choose whether a tagged item appears on your profile.
This short video demonstrates why tagging without permission isn't such a great idea - click here. You can also visit our dedicated page to check out how to set privacy and security settings for many social networks here