In baseball, VORP (value over replacement player) demonstrates a player's contribution to their team, as compared to an imaginary "replacement player." A replacement player is categorized as a less than average hitter and average fielder. VORP is used by many clubs to gauge a player's importance to his team, because it uses actual stats, and not projected or assumed stats.
The replacement player is assumed to be below average because it is assumed that the team would try to replace the player at a minimal cost, which is also called "freely available talent." Other statistics focus on comparing players to a league-wide average, which is great when comparing one era to another, but a league-average comparison loses steam when considering a player's value to his team. Baseball is a sport that must have a loser for there to be a winner, and the winner scores more runs, and every contribution counts. For example, if after 80 games, John Doe has a VORP of +20 runs, then he has contributed 20 more runs than his theoretical replacement would, and this figure will go up and down throughout the season. In general, a replacement player is assumed to reach 80% of the league average in any statistical category.