Ice beer involves lowering the temperature of the final beer until ice crystals form. Since alcohol has a lower freezing point and doesn't form crystals, when the ice is filtered off the alcohol concentration increases. It tends to have less character than other beers because the yeast cells/protein particles get filtered off with the ice.
Ice beer became an essential part of a brewer's portfolio, as each company sought to expand its market by imitating the hugely successful introduction of 'lite' beers. In their rush to add an ice beer to their portfolio many companies simplified the brewing process, freezing the beer until ice crystals formed, but then allowing the crystals to melt before continuing with their normal brewing process. This undermined the image of the beer for many consumers, and limited its retail success.
Ice beer is rumoured to have developed by accident in Germany during Oktoberfest celebrations with bock beers which are traditionally brewed for Spring. A particularly cold year froze the beers and and a new taste was noticed by the drinkers. These were called Eisbocks. However in its current form, ice beer developed from the fruit-juice industry which used to freeze orange juice concentrate to reduce shipping costs.