What is this cloud formation?
This a type of arcus cloud called a shelf cloud and is caused by moist air rising over cool thunderstorm outflow.  These clouds can look quite dramatic but do not pose any threats themselves.  However, they can signal the presence of strong winds behind the gust front.
These clouds can often be associated with strong convection -- squall lines, supercells, etc.  These storms often have strong inflow coming from ahead of the storm supplying the storm with moisture.  The storm is producing rain-cooled air and this dense air wants to spread out along the ground.  If the outflow becomes too cool (relative) or the inflow weakens, the storm outflow can race ahead of the storm.  This will produce strong winds as the gust front surges ahead and if the inflow is moist enough it will produce a cloud as it rises up and over the cool outflow.  This cloud marking the gust front as it the rises up and over is a shelf cloud.
These definitions are taken from a NWS storm spotter glossary:

Arcus - A low, horizontal cloud formation associated with the leading edge of thunderstorm outflow (i.e., the gust front). Roll clouds and shelf clouds both are types of arcus clouds.
Shelf Cloud - A low, horizontal wedge-shaped arcus cloud, associated with a thunderstorm gust front (or occasionally with a cold front, even in the absence of thunderstorms). Unlike the roll cloud, the shelf cloud is attached to the base of the parent cloud above it (usually a thunderstorm). Rising cloud motion often can be seen in the leading (outer) part of the shelf cloud, while the underside often appears turbulent, boiling, and wind-torn.