When you use a hashtag (#) before a word on a Twitter posting, it helps others find that topic.
Use hashtags to help in your job search, to see how and where others are discussing a certain topic.
You will quickly see the keywords most people are following (you can check that at Hashtags.org). These are the keywords that will let you connect with more people.
If you use them, you can build a larger network more quickly on Twitter, and you network will be composed of people with interests closely aligned to yours.