A tag is keyword or term that describes a blog post, in this context, allowing it to be found through browsing or searching. It is usually a word or phrase personally chosen by the blogger who generally has their own system of tagging that they follow.
The idea of tagging has been around since before the twentieth century, pre-dating computers. In this context tagging referred to the categorisation of papers and edge notched cards (an obsolete information storage technology). It was also used in libraries from the 1930s to help sort and categorise books. However, the tagging we know today became apparent through the invention of the World Wide Web. Early websites and databases created tagging as a way to browse and find content that they required. In 2003 a social bookmarking site called ‘Delicious’ started allowing its users to tag their bookmarks in an effort to simplify the process of finding them in the future. A few years later Flickr introduced tagging to its service, allowing the tagging of photos which helped to make the content posted highly searchable. The user-friendly nature of tagging then allowed it to become highly popular, with many sites then introducing it such as Youtube and Technorati. Following these introductions it then became a huge part of online categorising and searching, simplifying the process or sorting and browsing content.