Four couples (a square) perform intricate moves in response to rapid and (somewhat) unpredictable commands from a caller. The basic moves form the words of a language of dance and, like the words of a language, must be practiced until they are completely automatic.
Square dancing requires total concentration on what the caller is saying and on the position of the dancer in the square. You cannot dwell on your personal problems while square dancing. Even folks who are convinced that they can drink and drive know very well that they absolutely cannot drink and square dance. A healthier activity would be hard to imagine.
Many dancing activities use dance contests to provide "quality control". In square dancing, contests are extremely rare because the quality control is built in to this cooperative activity. Each dancer depends on seven other dancers to execute the correct moves with precision.
A square dance is made up of a series of sessions called tips. Each tip consists of two parts, a patter call and a singing call. The patter call consists of a rapid-fire sequence of spoken calls which lead the dancers through intricate and unpredictable patterns. There is usually background music to provide a basic beat, but the emphasis is on dance as an intellectual exercise. In the singing call, the moves are much more predictable and usually consist of sequences practiced during the patter call. The caller usually sings a popular song with the calls interspersed between the phrases and the emphasis is on dancing to the music.