A shear wall is a specially constructed wall made to withstand lateral (or horizontal) forces being exerted upon a structure from the effects of high winds or earthquakes.  The builders insert special bracing (diaphragms or panels made of metal, plywood, or PVC) that is anchored to a sill plate at certain points along a horizontal wall to give the wall greater stability.  This allows it to flex and bend more more easily than a conventional wall, but still holds it intact enough to not fall or blow down.  Sometimes a shear wall is made of specialized concrete or reinforced masonry anchored to a plate for stability.  Whatever the material, the principle is the same:  to keep the building more stable from side to side. 

Another important fact about shear walls is they usually have fewer openings in them because this is considered to weaken them.  You may see a long wall along the back of a home or building with very few windows or doors.  This is usually a shear wall.   

Certain areas of the country that are more subject to natural disasters (hurricanes, tornadoes, and earthquakes) have very strict building codes in regards to the construction and installation of these types of walls.  And while these walls are made for greater stability of the buildings during a natural disaster, there do not make them indestructible!  The basic idea of a sheer wall is to keep the building together as long as possible.  Many lives have been saved as a result!