What is the difference between tag and hashtag?

The definition of Hashtag :
(on social-networking websites) A word or phrase preceded by a hashmark (#), used within a message to identify a keyword or topic of interest and facilitate a search for it.

The definition of Tag :
Keyword description to identify images or text within their site as a category or topic. Web pages and blogs with identical tags can then be linked together allowing users to search for similar or related content. If the tags are made public, online pages that act as a Web-based bookmark service are able to index them. Tags can be created using words, acronyms or numbers.