In Twitter, what does putting a # sign before a word mean?

[strawberries]

These are usually at the end of Twitter messages. Here are some I've seen: #politics #usa #yss

Here's an example of one message:
http://www.reuters.com/article/worldNews...ZR20090411 #pman #cnn #abc #rai #bbc #usa #un #uk #ue

[volleyball213]

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://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.