No matter where you post your thing, you just have to add a hashtag to the post, and it automatically joins the group of posts that are similar to yours. Hashtags not only have the function of aggregating similar content, but have also been used to measure the popularity of topics. That explains the meaning of the buzzwords "trending topics". The more a hashtag is posted, the more people get to know about it.
Hashtags.org is a website dedicated to show a panorama of hash tags worldwide. The front page brings the most popular hash tags currently, and the portal also has an extensive archive of information about how to use this tool. To conclude, I would like to leave you with what is, in my opinion, one of the smartest ways to use hashtags ever. Jimmy Fallon is using the word hashtag to name a sketch in the Late Night Show. Every now and then, he posts an unusual hashtag on his Twitter account and lets people create the content for the hashtag. They all become trending topics instantly and that is helping Jimmy Fallon to solidify his place as one of the most interesting talk show hosts nowadays. If you do a search for "hashtags" on YouTube, for example, the first 15 links that appear are to Jimmy Fallon's show. So here's my hashtag for you, Jimmy: #goodjob