Short answers, pulled from the story.
The first direct detection of gravitational waves occurred on the 14th of September 2015, when LIGO's detectors in Livingston, Louisiana, and Hanford, Washington, registered a signal designated GW150914. The LIGO-Virgo collaboration announced the discovery on the 11th of February 2016.
GW150914 was produced by the merger of two black holes with masses of 29 and 36 solar masses, located about 1.3 billion light-years away. During the final fraction of a second of the merger, the event released more than 50 times the combined power of all stars in the observable universe, with energy equivalent to three solar masses radiated away as gravitational waves.
The 2017 Nobel Prize in Physics was awarded to Rainer Weiss, Kip Thorne, and Barry Barish for their role in the direct detection of gravitational waves. In 1993, Russell Alan Hulse and Joseph Hooton Taylor Jr. received the Nobel Prize in Physics for the indirect evidence provided by the Hulse-Taylor binary pulsar.
LIGO uses laser interferometry to measure tiny changes in the relative length of two perpendicular arms, each 4 kilometers long. A passing gravitational wave stretches one arm while shortening the other; even the strongest waves shift the arm length by no more than roughly 10 to the minus 18 meters, far less than the diameter of an atomic nucleus.
The first indirect evidence came in 1974 from the Hulse-Taylor binary pulsar, discovered by Russell Alan Hulse and Joseph Hooton Taylor Jr. Timing observations showed the orbital period decaying at a rate matching the energy loss predicted by general relativity for gravitational radiation, confirmed to within 0.2 percent after more than 30 years of data.
On the 17th of August 2017, LIGO and Virgo detected GW170817, the first observed inspiral of a binary neutron star system. The signal lasted about 100 seconds and was followed 1.7 seconds later by gamma-ray burst GRB 170817A. Seventy observatories joined in follow-up observations, identifying a kilonova in galaxy NGC 4993, about 40 megaparsecs away.