"ECW: One Night Stand" was an annual pay-per-view wrestling event held from 2005 to 2008. The WWE rebranded this event in 2008 as "Extreme Rules" under their own acronym.
Extreme Championship Wrestling, or ECW, was a wrestling enterprise that went bankrupt and folded in 2001. The WWE purchased its assets in 2003 and decided to relaunch the brand in 2005. The first "One Night Stand" was the event that kicked off the WWE's revival of ECW. The ECW was distinctive from the existing WWE brand in that it had more of a "hardcore" approach with less in the way of in-ring rules, more use of weaponry and "foreign objects" and more blood (both real and fake).
As with most WWE pay-per-view events, One Night Stand featured a main event match as well as an undercard of several other matches. The main event in 2005 was a tag-team match featuring The Dudley Boyz versus Tommy Dreamer and the Sandman. The main event in 2006 was John Cena versus Rob Van Dam for the WWE championship.
In 2007 the ECW acronym was dropped from the title. The main event in 2007 was John Cena versus The Great Khali for the WWE championship. In 2008 the main event was Edge versus the Undertaker for the World Heavyweight Championship.