Like most things that are distinctly American, IPAs were not born here. They started in England, with additional hops being added as a preservative to beers that were being shipped to hot climates. Including, of course, India. The elevated hop levels provide the bitterness that's become the calling card of IPA beer. While varieties vary from region to region and brewer to brewer, IPAs will always have a hoppy bitterness and high alcohol levels.
Over the last few decades, IPAs have become the unofficial mascot for beer enthusiasts across the US. Every microbrewery has at least one, and often more as a nation of hop heads grows seemingly by the day.