The Kepler satellite was launched in March 2009 and has since June 2009 been sending data down to Earth. It has been pointing at only one portion of the sky the entire time looking for variations in the brightness of stars. The image below shows the field of view of Kepler.

Kepler Field of View. 42 CCDs each with 2200 × 1024 pixels are used in order to get a large field of view. Credit: NASA

When an exoplanet passes infront of a star a small portion of the light is blocked out. Studying how the light is being dimmed gives astronomers a wealth of information about exoplanet candidates. I say candidates as a dip in the light curve alone is not enough to confirm the existence of an exoplanet. Binary and variable stars are examples of what is known as false candidates.

To quickly learn more about the transit method the following video might be of interest: The Exoplanet Transit Method – The Method – Part 2