After 38 games in a Premier League season (a result of playing each team twice) the bottom 3 teams who accumulated the lowest point totals are automatically relegated to the second tier of English football, known as the Coca Cola Championship (Or old Division One during EPL formation and back before the EPL was formed, known as the Second Division)
The Championship is based on 46 league games, a result of all 24 teams playing each other twice (obviously they can't play themselves twice, hence 46 games, not 48)
The team finishing highest in this league is AUTOMATICALLY PROMOTED TO THE PREMIER LEAGUE AS CHAMPIONS.
2nd place is also AUTOMATICALLY PROMOTED
From 3rd place down to 6th (a total of 4 teams) enter into the Championship Play-offs, where each team will play another in a round robin format, usually 3rd place faces 6th, and 4th vs 5th place.
The winners of either match (played over two legs) faces each other in the Championship Playoff Final, held at the New Wembley stadium. Usually it is the penultimate game held at Wembley and on the English Leagues Football calender (European matches excluded)
The winner of the match, decided by either 90mins result, extra-time or penalties, is promoted to the Premier League AS CHAMPIONSHIP PLAYOFF WINNERS.
The same system applies for each League in English Football, starting from
Premier League 1st tier
Championship 2nd tier (3 Promoted, 3 relegated)
League One 3rd tier (3 promoted, 4 relegated)
League Two 4th tier (4Promoted, 2 relegated)
Blue Square Premier and below are classed as non-league
Manchester City fan, i know a fair bit about promotions and relegations between the leagues.