All-Pro is a term mostly used in the NFL for the best players of each position during that season. It began as polls of sportswriters in the early 1920s. There are also 2nd Team All-Pros which connotes the runner-up at each position.
The Associated Press NFL All-Pro Team
The Associated Press NFL All-Pro Team is an annual selection of the best players in the National Football League (NFL) by position as selected by a national panel of media members of the Associated Press. Unlike selection to the Pro Bowl (all star game), votes are cast for outstanding players by position without consideration for whether the player competes in the American Football Conference (AFC) or National Football Conference (NFC). The Associated NFL All-Pro Team is the longest running selection awards program in existence
The First Team consists of the top one or two players at each position; the Second Team consists of the runners-up at each position. One player is selected at quarterback, fullback, tight end, center, punter, place kicker, and kick returner, while two players are selected at running back, wide receiver, offensive tackle, offensive guard, outside linebacker, inside/middle linebacker, defensive end, defensive tackle, cornerback, and safety.
The Associated Press and its Associated Press NFL All-Pro Team is the "All-Pro team" that is of most note, likely because it has been consecutively chosen since the 1940s. The United Press International All-Pro poll began then as well, yet beginning in 1970 UPI began to choose All-Conference teams, that is one each from two conferences in the National Football League, the AFC and NFC. UPI last chose those teams in 1996. The Newspaper Enterprise Association began choosing All-Pro teams in 1954 and they ran though 1996. The Pro Football Writers Association began choosing All-Pros in 1966 and continues today. In the early 1990s they began publishing their All-Pro team in Pro Football Weekly.
In the case with the press polls, as are mentioned above, they are simply a poll of writers who are asked to select the top players at each position on a football squad. The votes are tallied and were published in the various newspaper syndicates, like the AP, UPI, NEA, etc.
The Sporting News
The Sporting News published All-Conference teams beginning the 1950s. In 1980 it began choosing an All-Pro team, rather than two All-Conference teams. Since its teams are published in Total Football: The Official Encyclopedia of the NFL, they are recognized by the NFL and the Pro Football Hall of Fame.
Newspaper Enterprise Association All-Pro Team
The Newspaper Enterprise Association All-Pro team was different than the press polls. It was created by Murray Olderman in 1954 purporting to be the "Player's All-Pro Team" as it was a poll of NFL players themselves. This poll last published in 1997.
Pro Football Writers Association
This All-pro team began in 1966 and is still released each year. Now, it is released in Pro Football Weekly. It is a poll of the members of the Pro Football Writers Association and is considered a "press poll". Additionally, Pro Football Weekly has selected All-AFC and All-NFC teams since 1970. Those are the choices of the editors and writers of the magazine.
United Press International
Also a "press poll". It began in the 1930s and continued until 1969. In 1970 UPI began selecting All-AFC and All-NFC teams. Those ran though 1996.