IPA- a pale ale with more alcohol and hop bitterness to prevent infection from long storage. Originally made by the Brits for their soldiers stationed in India.
IIPA- an IPA with a higher alcohol content, hence the name Imperial.
EPA- a pale ale of modest bitterness made with English yeast and hops, generally more earthy tasting.
APA- a pale ale of modest bitterness made with American yeast and hops, generally more citrus tasting.