Porter is simple a style of ale brewed with darker malts. The line between porter and stout can be blurred, but generally speaking if a maker has both the porter is just a lesser version, perhaps a bit dryer and not usually heavily hopped.
Stronger porters are usually called Baltic Porter, and can be over 7%.
Usually you won't see as much creativity with porter as you would with stout though. Things like lactose, fruit etc... are usually reserved for stouts, although I have seen coffee and chocolate in porters.
-spence
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