LinkedIn is a professional network with over 60 million users worldwide. Whilst not as popular in Australia as other places, it is quickly gaining acceptance and numbers in Australia. The advantage of LinkedIn over other social networking sites is that it is focuses on business relationships rather than personal relationships.
If you search on your name, your LinkedIn profile (if you have one) will in most cases rank #1 or #2 in Google, so if people are searching for you it provides an invaluable opportunity to showcase your qualifications, skills and knowledge.
The benefits of LinkedIn are probably best explained by LinkedIn themselves.