Bow Echoes, Derechos and Mesoscale convective vorticies
A bow echo is a small (typically 20 - 120 km long), bow-shaped system of convective cells that is generally well-known for producing long swathes of damaging surface winds. They are observed both as isolated convective systems and as sub-structures within a much larger convective system, and generally last for less than 6 hours. 
 
Bow echoes form when strong winds help the line of thunderstorms to surge forwards in the middle of the line. This occurs often, due to the coupling of strong rear-inflow jets and/or the production of a strong downburst in the middle of the line. The jet/downburst will push the storms ahead of it more rapidly than at the edges, where there is less momentum. This creates the classic "bow" shape when viewed on radar - in fact, squall lines often display this shape. Generally, bow echoes have a well-recognised evolution, starting as a strong isolated cell or a small line of cells that evolve into a symmetric bow-shaped segment, and ultimately into a comma-shaped segment over several hours. 
A mature bow echo in Minnesota. 
From: http://pauldouglasweather.blogspot.co.uk/2012/07/hottest-day-of-year-100s-today-weekend.html
 
Interestengly, since bow echoes propagate at the speed of the cold pool they produce, their propagation speed is often much faster than nearby convective cells or systems, since their pools are usually very strong. The extreme intensity of bow echos is partly due to their relatively small size - the smaller distance between the bookend vorticies enhnaces the focusing effect flow between them, which can strengthen the rear inflow jet. 
 
A derecho is a large straight-line wind storm that is associated with a very severe bow echo. Derechos can be amongst the most damaging convective storm becuase they produce very damaging winds over a large area. A derecho may occur if the rear-inflow jet descends to the surface or if there is a widespread downburst event within the thunderstorms. A typical derecho may produce winds of around 80 mph, but extreme derechos can produces widespread winds of over 100 mph.  
Composite radar image of the track of the June 29, 2012 derecho, which impacted Indiana, Ohio and West Virginia. 
Credit: NOAA
Conceptual model of the formation of a bow echo. The black dots labelled "DB" are downburst locations, with the colours indicating radar reflectivity intensity. 
From Markowski and Richardson textbook: Mesoscale Meteorology in Midlatitudes, 2010). 
The conceptual model of a bow echo formation, above, indicates the presence of a "hook" cloud head and an eventual comma radar echo. The rotating head is called a bookend vorticy or a mesoscale convective vorticy (MCV). Bookend vorticies often form on the tips of bow echoes becaue of the shear vorticity created here between the forward-moving storm cells and the relatiely stagnant surrounding atmosphere. The bookend vorticies themselves can enhance the rear-inflow jet and initiate the bowing process. A MCV will form after several hours, when the effect of the Coriolis effect can cause the cyclonic vortex (in the Northern Hemisphere) to become dominant - this is because air circulates in a cyclonic motion (anticlockwise) around a low-pressure centre in the Northern Hemisphere. 
 
Bookend vorticies associated with a bow echo. The northen, cyclonic vortex is better defined and more dominant here. 
From http://www.wunderground.com/blog/beell/weather--may-26
There are two main types of derechos - progressive and serial derechos. Progressive derechos are usually characterised by a single bow-shaped system that propagates north of, but parallel to, a weak east-west oriented stationary boundary. Serial derechos are composed of a series of bow echo features along a squall line (called line echo wave patterns). This is usually located within the warm sector of a synoptic-scale cyclone. Essentially, the movement of serial derechoes is dominated by the advection of wind from the cyclone, whereas for progressive derechos, the movement is determined almost entirely by the cold pool propagation. 
 
Schematic of a serial derecho (with multiple line echo wave patterns). 
From: http://www.spc.noaa.gov/misc/AbtDerechos/derechosqlns.htm
 Schematic of a progressive derecho.
From: http://www.spc.noaa.gov/misc/AbtDerechos/derechosqlns.htm