Tagging seems like the same as typing in a keyword into Google search and finding something related to that keyword. The difference is "Tagging" or attaching meanings to words is informal...where as Google is more of the opposite.
I have personally used Tag before but in Facebook people can be "tagged" in pictures and assigned some sort of definition. If that make sense.
Many of the assignments can be used in libraries and this is no exception, though I probably would not recommend using Tag because of the inability to control the what is tagged and what is not.
If these past few paragraphs seem a little confusing, it is because I am not 100% confident that I know what Tagging is...though I am familiar.