LinkedIn is the world's largest professional network with over 175 million members in over 200 countries. It allows users to build professional networks, research companies and employees, find jobs, and create online profiles that function as resumes. The document provides tips for optimizing LinkedIn profiles, such as including a photo, using relevant keywords, tagging skills, and linking to other professional websites and content like presentations. It also describes additional LinkedIn features for networking, including connections, groups, polls, answers, viewing profile views, suggested connections, and job search tools.