Ginger Harper   Long answer here:

A hashtag is a tiny piece of metadata that allows users of microblogging sites or social media sites to find or follow specific keywords or themes or content.

    For example, on Twitter, if you enter the hashtag #realestate  into the search box up at the top right side, then Tweets with that hashtag will show up in the stream that appears.

    Click this Link to see Tweets that are using the #mortgage hashtag (it may require you to log in).  If you get a blank page, enter #mortgage in the search box, then scroll across and click on Live so you can see real time Tweets on #mortgage.

   The same principle applies to Instagram, Google Plus, and other sites that recognize hashtags. 

   BTW, #mortgage and #realestate are not getting much activity right now (late Saturday night) because it is after business hours.  Given that today was a Primary Election Day, try searching a political hashtag like #Trump or #LosewithCruz.

    Be sure to type hashtags into your own comments on sites that recognize hashtags!

Candice A. Donofrio has the clever answer! And Fred Griffin gave you the in-depth version.

It is a symbol that precedes a search term for convenient curation/cataloguing of info because we are
#evilgeniuses :)

Fred Griffin, you are the best.  Great explanation. 

See answer by Fred Griffin 

Thanks for the question and thanks for the answer Fred Griffin 

Thanks for asking the question as I picked up a couple of tips.

Let's go "OLD"  school.   Remember the yellow pages ?   When you opened the book, you went to a section titled "Real Estate" to find the contact information for other agents.

Fast forward to today.   It's like an address in cyberspace.   You might go to "#realtor" to find other agents.   Same concept, just modernized.

Commonly it is used for people of like mind to find each other or for strangers to enter into a specific cyberspace community they relate to or want to interact with.   If I was talking about or trying to reach you, I might go to "#gingerharper".

I liken it to being a tag that makes searching for posts on topics easier.

A tag like #openhouse during an open house advertisment helps people searching for that phrase find the article.

as usual, Fred did a great job of instructing everyone....

I still like to call it the "pound" sign

 It is a twitter thing and I don't do Twitter. I think it's for the birds. But then, that is my two cents worth.

See https://en.wikipedia.org/wiki/Hashtag

It can be compared to using a yellow highlighter. Think of the trigger words for your business. If you are on twitter, instagram, Google+ you want to write about your hot topics so buyers or sellers can find you. In a sentence Ginger Harper you would say, #Southport #NC #sea #coast #gorgeous #home for #sale. The number sign will lead others who search for one of the words to your user account. Regular "tags" also work in blogging. Photos should be tagged for image showing, your topic, and perhaps your name. What are the 7 words a buyer or seller will search on Google or Bing... to lead to your name? Focus your blogging, pinterest, instram, G+ or..... million other sites on those words. This will make individuals find you. 

adding it tags your word or word combination and help others to find it.

parked and reading

I know what it is but I can't explain it

Ginger Harper it's the Social Media search term - perhaps that's the simplest explanation that I can give.

Of course there as so many good answers here....

I learned a lot reading through these answers and feel very fortunate to have access to so much great information here.  Thanks for asking this Ginger Harper.

Stopped to see how ya'll use the #

It began on twitter and then facebook picked it up.  It becomes a "trending" link.

It points out the most important word in your writing. What you want noticed.

Seems to be a universal signal to search for the term that follows it.

Thanks

The ans are informative.

I don't totally get it either #hatethehashtag

I know it's used in twitter, but I really don't know what it means, and I don't really use twitter.