Here is a comprehensive summary of the Wikipedia article on "All-Pro":

All-Pro is a prestigious honor in American professional football, designating the best players at each position during a given season. Historically, these teams have been assembled through press polls of sportswriters since 1923, with organizations like the Associated Press (AP) being the most widely recognized selector today, having begun its annual picks in 1940. While the NFL itself compiled official "All-League" lists from 1931 to 1942, it later abandoned the practice.

All-Pro selections are generally considered more exclusive and prestigious than selections to the NFL's Pro Bowl, as they involve fewer players and typically feature distinct first and second teams chosen without regard for conference affiliation. Other notable press organizations, such as United Press International (1940s-1997) and the Pro Football Writers Association (since 1966), have also contributed to identifying these elite athletes.