Nielsen is a global measurement and data analytics company that provides market research, insights, and data about what people watch, listen to, and buy. It is a big name in ratings, as it is the most widely used source of television ratings in the United States. Nielsen also provides ratings for radio, online, mobile, and out-of-home media. It is a leader in providing consumer behavior insights and analytics, which are used by companies to make informed decisions about their products and services.