Tags are the tools of taxonomy.
These are traditionally keywords attached to a blog post, bookmark, photo or other item of content so you and others can find them easily through searches and aggregation. Tags can usually be freely chosen - and so form part of a folksonomy - while categories are predetermined and are part of a taxonomy.
Using taxonomy means that you are not inventing the hierarchy that people use to find you, rather you are tapping in to the existing usage and making your content more powerful.
Tags are a keyword or term associated and assigned to an item of content (i.e. blog post, video, photo, etc.) by that content's author. . Tags are usually added to an item of content to enhance search engine optimization and make it content easier to organize and find. Often many posters do not tag content due to laziness.
A smart marketer plans on using tags effectively as a calling card driving people towards his products and output.
Budget: $100 - $350 monthly