Tags are a way to organise lots of information. They are simply short descriptions for a subject. They are like keywords, or a label for a category. Tagging is known by a few different names, such as content tagging, collaborative tagging, social tagging and even the scientific-sounding “folksonomy.” In general tagging can be defined as the practice of creating and managing labels (or “tags”) that categorize content using simple keywords.

The video below explains tags in terms of the social bookmarking tool – “delicious” – personally I prefer using Diigo but the video gets the point across