Clickers are "personal response systems" that work much like TV remote controls, allowing each student to respond to questions posed in class by "clicking" a button that transmits their response to a receiving station. These come with additional tools that aggregate all the responses and generate graphs that can be displayed for the class to see.
For more information about how clickers can be used, see:
Middlebury has a number of these clickers available for use in classes. Contact Shel Sax for more information about reserving these for your class.
Clickers in a large lecture class (Catherine Combelles)