The hash mark (#) before a word in a post allows you to tag that post for that word. However, in order to get tracked via a hash tag, you need to opt-in and follow http://twitter.com/hashtags. Once you’re following Hashtags, every time you make a post in Twitter and tag it with a hash mark like so: #iPhone, it will then show up as a real-time post on http://www.Hashtags.org.
If you then visit Hashtags.org, you can click on any tag and it will show you all of the posts that have been tagged with that keyword.
I use the term “keyword” with a grain of salt since some of the tags aren’t keywords in the sense that you know them related to search engine optimization, because some of the tags are completely non-sensical in nature. And although you can get a complete list of words that have been tagged on the site, there is no way (at least currently) to search for a specific tag.
However, it is possible to view all tweets (posts made to Twitter) that contain a specific tag by modifying your URL a bit like so: http://hashtags.org/tag/iPhone.
You can also subscribe to any updates that contain a tag by posting “follow #tag” (without the quotes) in Twitter.
Since Twitter only allows you to post 140 characters at a time you can see how you’d have to be pretty selective when using the hash mark - although one character doesn’t seem like much, you’d be surprised how much you actually need that one additional character when trying to get a point across in your Twitter post.
Here is yet another site that you can use that makes use of hash marks. This site allows you to see who has been using the hash mark you enter within their posts and how many times they’ve done so. Note that when you visit this site you don’t have to enter the hash symbol (#) in the search box.