Sometimes called the Beijing or Peking form for its place of origin, the simplified 24-form tai chi is a short version of tai chi composed of twenty-four unique movements.
The form was the result of an effort by the Chinese Sports Committee, which, in 1956, brought together four tai chi teachers:

Chu Guiting, Cai Longyun, Fu Zhongwen, and Zhang Yu.

to create a simplified form of tai chi as exercise for the masses. Some sources suggests that the form was structured in 1956 by master Li Tianji.  The creators truncated the traditional family style tai chi forms to 24 postures; taking about six minutes to perform and to give the beginner an introduction to the essential elements of tai chi, yet retain the traditional flavor of traditional longer hand forms (in general, 88-108 postures).

The 24-form is most likely the tai chi form with the most practitioners in China and the world over.