A hashtag is a word or string of words prefaced with a # sign that tells what your post is about. They are highly used on social media platforms such as Twitter, Facebook, Instagram, and Google+. It is used as a label for the content of your post. Searching for hashtags will likely bring up content with relevant content. For example, if I was posting about weather in Boston, I may use the hashtag #BostonWeather. If I post regularly about the topic of SEO I may utilize #SEO or #SearchEngineOptimization for many of my posts. Typically, hashtags do not use spaces and are one long lumped together phrase if it includes multiple words.
Hashtags often may be used by companies for contests, to conduct Twitter chats, or used to aggregate content from an event. For instance, if I was attending the upcoming PAX East in Boston, MA, I may use the hashtag #PAXEast to share my thoughts with others attending the event.