Hash tags look something like this: #marketing (that’s the hash tag for marketing).
When you put a hash tag on a tweet it becomes much easier to find for people searching on a particular topic. Alex Mandossian uses #tss when he and his clients (and prospects) discuss his Teleseminar Secrets program on Twitter.
#tcot hash tag is the most popular
Here is a video from social media expert Mari Smith where she explains in detail how to use hash tags and how to track popular tags.