A tag is basically a keyword. It's a non-hierarchical method to describe something. For example, take an article that is about security and cloud computing. It would be tough to classify such an article in just one category. Using tags, you can describe that the content is about both topics and differentiate it from, say, an article on cloud computing and performance.

Here are some examples of hashtags:

• #tech
• #CloudComputing
• #FightClub
  

It's also common to hear the term at conferences. Often the moderator will announce a hashtag so that all those tweeting can see the thoughts of the other audience members.