LinkedIn is an online network of more than 11 million experienced professionals from around the world, representing 150 industries.
When you join, you create a profile that summarizes your professional accomplishments. Your profile helps you find and be found by former colleagues, clients, and partners. You can add more connections by inviting trusted contacts to join LinkedIn and connect to you.
Your network consists of your connections, your connections' connections, and the people they know, linking you to thousands of qualified professionals.

http://www.linkedin.com/