Twitter know lets you see what the trending topics are on your Twitter home page. However, many of us do not have a clue what these topics actually are.

What the Trend? to the rescue! This application (by Matt Mayer) explains what the topics are, when they started and the last time they were tweeted about. If you click on a trend, you can see the tweets, any news or photos (from Flickr) about the subject as well.

You can also follow @whatthetrend on Twitter.

Twitter does have search available since they bought the site Summarize. To access the Twitter Search, go here . The page allows you to search by topic, and has a list of the current trends at the bottom.

Someone figured out that if you put the # in front of a term, people could find it quickly here. Voila! The hashtag was born. (The term may have originated from somewhere else, I’ll look into that.)

So, when a number of people are at a conference, such as BlogWorld Expo, they try to agree on a single tag and then place it somewhere in their tweets, like this #BWE08. This allows others to find their comments under one search instead of having to do variations on the name of the conference.

Some people have gotten a tad crazy with hashtags (IMHO), tagging everything they think is useful for others. A number of people have put together online parties by using the hashtag so that others can see who is online without following them. Warren Whitlock put such a party together over New Year’s Eve by using the tag, #Happy09.