An electronic metering device attached to a TV set to measure tuning status (set on/off, channel, time, and duration of tuning) as well as demographic data (who is watching). Household members and their guests push buttons to identify themselves. The People Meter is used by Nielsen Media Research to measure nationwide audience measurement in the U.S. and Canada.

Source:
http://www.nielsenmedia.com/glossary/
http://www.arbitron.com/downloads/terms_brochure.pdf