Aperture is a lab that employs various psychotic and sentient robo-personalities to perform various forms of stress testing upon human subjects, willingly or unwillingly. All human subjects are given a rather unique piece of equipment that allows them to create wormhole-connected portals on certain surfaces. This unexpected behavior is considered highly irregular to most human minds and so the tests, which physically stressing, are almost completely mentally exhausting 100% of the time.
Subjects are goaded on by promise of cake, which is a lie.
The Aperture labs are run by pan-dimensional beings which in our physical world appear to resemble mice.
Lightroom is their competitor who, for lack of a suitably mind-bending tool and psychoticly sentient systems, failed to garner a contract with Valve.