In an attempt to track program listening for advertisers, Robert F. Elder and Louis F. Woodruff developed and sold an audience measuring system to the Nielsen Company, in the 20's and 30's, and released statistics in 1947 under the Nielsen Media Research company. It was intended to determine what people were listening to on the radio so that networks could charge advertisers based on the audience and the popularity of the program. It moved to the television broadcast section in 1950.

At the start of the program, volunteers kept a diary and mailed their information to Nielsen Media Research. Today, there is a "Set Meter" that can be connected to a TV that tracks viewing times and length.

With the invention of TIVO and Web-Streaming, the rating system has been unable to come up with a solution to tracking program watching and popularity. Many feel the system is archaic and should be retired.

Nielsen Ratings have put many TV programs to an early death that performed very well in syndication. Star Trek is a good example of this.

More Info: en.wikipedia.org