On Wednesday, the DOW fell over 150 points in the morning, in large part due to investor confidence being severely shaken by the massive selloff in China, and the aftershocks of the Greek referendum.
Investors around the world are reeling form these two combo-shocks, we have talked about some of the causes of the Chinese crash Here, but CNN has also published a brief explanation Here
As for the Greek crisis, the causes and future path are a wide path of economic issues, the Wall Street Journal has done a good job of putting together a point-by-point summary here for anyone who needs a primer on what went, and continues to go, wrong.