The word brutalist was assigned to certain structures as a criticism, rather than a term that architects used for their own designs. Brutalism was an architectural style that occurred mainly between the 1950s - 1980s.
The use of concrete is often a characteristic of brutalist structures, however, the use of concrete is not an all-defining characteristic. Rather, brutalist architecture seeks to use raw, unfinished materials. The use of facades and extra ornamentation is eschewed by a brutalist architect. Also the size, shape, and placement of windows will often be relatively small compared to the overall size of the building. Brutalist architecture quite often stands in stark contrast to the environment around it (whether it be nature or other architecture). Brutalism also has a reputation of not being received well by the general public and as a result, many brutalist structures will have facades wrapped around them, or they are in danger of being demolished. Here in the United States, brutalist structures are less common than in other parts of the world, but still you can find many of them as govt buildings and on college & university campuses.