What? Beer that is aged in wood to add flavours to enhance the taste. Most wood aged beers tend to be aged for between 60 and 90 days, although we do have some beers that have been matured in wood for a few years. The most common wood used is oak, as it is strong, and imparts desirable flavours on to the liquids it contains. Oak aged ales tend to be stronger in alcohol and flavour.
Where? As craft brewing has grown in the United States, more and more breweries have been experimenting with wood ageing. The same is starting to happen in the UK, Belgium, the Netherlands and other countries that have a strong craft brewing movement. Barrels add to the cost of production, so you tend to find it is only higher end beers that are barrel aged.
When? Coopering (the production of barrels) is one of the oldest professions. For example, the Romans stored wine in oak, not only because it was useful, but because it made the wine taste better. This would have been similarly true in the past with ale.
How? Wood aged beers tend to be stronger in alcohol and flavour, so should be savoured slowly.