Gravitational waves are best decribed as ripples in space-time. If you think about a boat travelling through the water, its movement will leave ripples or waves behind. In the same way, a mass moving through space - such as a star - will leave a kind of 'ripple' in the space time 'ocean' or fabric. (It can help to think of the space-time fabric as something like a big sheet of material, that will bend its shape if you put a heavy object in the middle of it. In the same way, if you drag an object along a taut piece of material, it will create waves or ripples in the material.)
What causes them is the interaction between two bodies or masses - for instance, two stars orbiting each other. As they get closer they send out waves of radiation, which get weaker and weaker as they move further away from the original mass (just as the ripples from a boat get smaller and smaller). However, as there is nothing to obstruct them the waves do eventually reach the Earth, where scientists are able to observe them.