What's the # for at the end of a text or whatever. ?

[Alexander]

Such as Twitter.

[Smilinguncle557]

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.