A wave in the fabric,not in a medium.
Gravity is not a force pulling through empty space. It is the shape of spacetime. When that shape is disturbed, the disturbance can travel.
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Hard · strain
A gravitational wave stretches space in one direction and squeezes it in the perpendicular direction. The dimensionless strain h is the fractional change in length: ΔL / L. For the waves LIGO hears, that is about one part in 1021. On four kilometres of arm, the stretch is smaller than a thousandth of a proton.
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Speed
In general relativity the waves travel at the speed of light. GW170817, a neutron-star merger with a gamma-ray counterpart, confirmed that the difference is at most a few parts in 1015.
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What radiates
Stationary mass does not radiate gravitational waves. A time-varying quadrupole moment does: a lopsided arrangement of mass, changing as it moves. Two black holes in orbit are a loud source. A perfectly symmetric collapsing sphere is silent.
We cannot see the objects collide. We can feel spacetime remember that they did.
Einstein (1916, 1918) predicted gravitational waves; the quadrupole formula and later full numerical relativity supply the waveforms. Direct detection: LIGO, 14 September 2015 (GW150914), announced February 2016.