The making of "ice" beers, in general, involves lowering the temperature of the finished product until the water in it begins to freeze and then filtering out the ice crystals that form. Since water will freeze before alcohol, the result is a higher alcohol content. The ice forms around yeast cells, protein particles, etc., which get removed as well, leaving fewer components to provide taste and character.

This process is not new to brewing, having been developed in Germany to produce "eisbocks", which were apparently produced by accident during the traditional spring celebration with bock beers.

In its current incarnation, the process is an offshoot of the concentrated fruit juice industry; it was developed by orange growers to reduce the costs of storage and shipping by concentrating the fruit juice through freezing and removal of some water. Labatt Breweries claims to have pioneered this process for brewing, and most of the large North American brewers quickly followed suit in the usual marketing frenzy.

The main difference between these "ice" beers and true ”eisbocks” is taste and character. Any beer brewed using this method will only be as good as the brew with which you start. In other words, if you start with a bland, flavour-impaired, adjunct-laden beer and remove some of the water, you end up with a bland, flavour-impaired, adjunct-laden beer with more alcohol. On the other hand, if you take a rich, malty, traditionally brewed bock and remove some of the water, you end up with an “eisbock”.

Source : FAQ of alt.drinks.beer