The second part is easy - all you need are two black holes next to each other. Yes, the very objects Einstein also predicted yet cannot be seen - black holes - are essential to finding the gravitational wave.
But back to the first question. Gravitational waves are ripples in space time. And what is space time? Well, that would be a combination of the regular three dimensions plus the fourth dimension of time.
And where did they spot these gravitational waves? They came from two intersecting black holes 1.3 billion light years away.
And how did we notice these waves? We measured them with special equipment illustrated below by Science magazine. That's right, we are measuring something 1/10,000th the width of a proton - don't try this at home.
I recommend two videos that explain this latest discovery, both from PBS's online series Space Time. The first was broadcast last year titled Have Gravitational Waves Been Discovered?!? and the second was just released titled LIGO's First Detection of Gravitational Waves! All your questions will be answered.
