IPA stands for India Pale Ale, and is a heavily hopped ale that was originally produced in the United Kingdom for export to it's colonies in India. Due to the long journeys required to transport such items, perishable goods like beer tended to spoil. In an attempt to stop this happening, brewers started to produce ale with a higher alcohol percentage, and with abundant hops. The increased alcohol content helped to preserve the beer, and the hops helped to cover up any spoiling.
In recent times IPA has seen a resurgence in the United States, with many different styles becoming popular whilst retaining the heavy hop usage and higher ABVs.