Clickers allow instructors to ask questions and gather students' responses during a lecture. Clicker systems are also commonly called Classroom Response Systems, Student Response Systems, or Audience Response Systems.
In clicker systems, each student uses a device (a "clicker") that looks like a simple TV remote to answer questions posed by the instructor in a specially-designed PowerPoint presentation. Summaries of student responses can be shown in real time to both instructors and students. Answers are also stored electronically for later viewing.
Ohio State has standardized on clicker devices and supporting software from Turning Technologies, LLC to ensure that that the OSU community receives a quality product and that students won't need to purchase multiple clicker devices for different courses.
For two video case studies of clickers in use at OSU, see Teaching with Clickers.