What is tagging?
Tagging is a mechanism for linking to relevant resources. Tagging is implemented in internet forums, blogs, collaboration systems (e.g., Wikipedia), and social networks (e.g., Flickr). The tag can be an in-text keyword or out-of-text keyword labelled by word or phrase. The in-text keyword tagging methodology focuses on some keywords in the content that may link to other resources. In contrast, the out-of-text keyword maintains tags out of the content body. Tagging in article sharing system is similar to keyword indexing of web search system. However, the tagging is focused on on-site retrieval particularly. The user specifies keyword and later the articles that are tagged with such keyword are retrieved.
You can use Zotero to tag articles automatically; although, it doesn’t work all the time (I think it depends on the page metadata).
Here’s an interesting video that describes tagging:
I hope you find it useful.