This is a pretty good reference piece from Mashable this morning. Plus, it links you to that hilarious Jimmy Fallon/Justin Timberlake video.
Hashtags are everywhere on social media, so you need to at least have a knowledge of them, even if you don’t use them too often.
BTW — Outside of the U.S., where # is generally called “the pound sign,” or Canada where it is generally called “the number sign,” # is generally referred to as a “hash mark.” In the U.K., of course, a “pound sign” would be this: £, which indicates their currency, the pound sterling.
So, there you have it: the international roots of the hashtag. Use it in #goodhealth.