What does the "#" sign mean on twitter?

[Don]

[The Amazing Sammy]

It's a hash tag. An easy way to organize a topic between users. You can use them at will. Use your own, or tags others have made up. It's pretty cool.

[Maria and George]

It means Discover.

[roundspoon664]

In Twitter, # called 'hashtag', such as #twitterer, #teamyuna, #newlook. It is a label on Twitter to aid searching and used to mark keywords.

For example, if you are posting a job on Twitter, you may add a #Jobs hashtag, so that your tweet would show up in searches for people who are looking for a job. Hashtags are not used often enough by corporate America on Twitter, as they can drastically increase the number of non-followers who read your tweets (which is one of the keys to get more followers).

Even though these work extremely well, you should generally limit yourself to using only 1 or 2 hashtags to each post, because excessive tags are annoying and a turn-off for most users.