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A gravitational-wave detector (aka GW detector, gravitational-wave observatory, GW observatory) is a device/facility to detect gravitational waves (GW detections). The current method of choice is a very large and very sensitive Michelson interferometer. It detects changes in distance between objects that are some distance apart (specifically, a pair of mirrors), the changes due to the modification of the scale of space (a modification inherent in the waves) by monitoring changes in the light travel-time between the mirrors. Current detectors have light-paths that are kilometers in length, and detect distance-changes that are substantially smaller than the width of an atomic nucleus. Given the movements are this small, the multiple separated detectors offer a huge advantage in distinguishing local movements (such as from a truck driving by, shaking a mirror) from the type that passing GWs would produce, which would show a matching pattern of changes nearly simultaneously even for detectors thousands of miles apart. The distant placement also helps in determining the direction of the waves' source, which travel the speed of light, and the detectors' differing attitudes (the directions of their light-paths) contributes to this as well because the direction of the waves' source affects their sensitivity to the wave amplitude (gravitational wave strain).
Current examples are the two LIGO detectors, and Virgo, which have been successful detecting recognizable gravitational waves and KAGRA, which became operational in February 2020. These are all on or under ground. Two concepts for similar but larger, more sensitive future ground detectors are Cosmic Explorer and Einstein Telescope. Similar space-based detectors using far longer lengths have been proposed but not deployed. They aim to detect waves of in a lower frequency-range. Two other types of detection are being attempted through analysis of more conventional EMR-observation are pulsar timing arrays (PTAs), and CMB studies. Another less-direct type of detection was achieved some time ago, the detection and measurement of an orbital decay of the Hulse-Taylor Binary that matches the predicted effect of gravitational waves.