LinkedIn is a professional social networking site that allows users to connect with colleagues, research people and companies, and share content. It can be used to deepen existing relationships, find new referral sources and connections, and uncover hidden relationships within one's network. The document provides tips for setting up a professional profile, getting connected with 50+ contacts in the first month, researching connections' connections, and spending 10 minutes per day on the site participating in groups and answering questions. LinkedIn can open doors to new opportunities by helping users find potential clients and speeding introductions to other professionals.