The earthquakes in this region are caused by the convergence of the Indian landmass with the Asian continent. India is being forced under Nepal and China, and the resulting thickening of the crust has lifted the Himalaya to form the highest mountain range on the planet.
For the most up to date information, consult the event page at the U.S. Geological Survey. Moment by moment blogging on the quake can be seen at http://earthquake-report.com/2015/04/25/massive-earthquake-nepal-on-april-25-2015/.
Update, 9:30 AM PST: More than a thousand people reported dead, but many more are missing. At least 10 climbers and sherpas are dead on Mt. Everest, due to an avalanche. It's been a bad year on Everest, as an avalanche took 16 lives about a year ago.
The latest consensus of the magnitude places it at 7.8. Magnitude is measured in several different ways (Richter, or Local magnitude is not a useful method for quakes larger than magnitude 6.5), and early instrumental calculations are often revised as more data is collected.