Have you been using Twitter lately?
Does the sign “#” familiar to the eyes of users out there?
I am pretty that symbol is very familiar to you guys… But if you have not yet started signing up there, try it and let the fun starts! 
So about this symbol, Robin Broitman said,
Twitter hashtags are a great tool for finding, following and participating in topical discussions relevant to you. Here I’ve put together a collection of articles with information and advice on using Twitter Hashtags.
If you are still left hanging, hold on tight with your mouse over on this link and discover the Secrets Behind Twitter Hashtags!