Aperture Science is the mysterious research corporation behind the plot of Portal.
According to its website, Aperture was founded by Cave Johnson in 1953 as a manufacturer of shower curtains. 26 years later, Aperture began development of the 'man-sized ad-hoc quantum tunnel through physical space with possible applications as a shower curtain,' also called the 'Portal.'
Aperture eventually created the Aperture Science Handheld Portal Device, and built the Aperture Science Enrichment Center to test it. They also created GLaDOS in response to the news that Black Mesa was working on a similar portal technology.
The game, Portal, is set mostly in the test chambers of the Aperture Science Enrichment Center. After escaping the test chambers (and near-death), the player finds that the Enrichment Center is deserted, with abandoned desks and disconnected phones. It is apparently run only by GLaDOS.