This is a Viceroy Butterfly. You can tell it is not a Monarch because of the curved, black line across the hind wings. Until recently, it was thought that Viceroys protected themselves by mimicking Monarchs, because Monarchs have a very bad taste. This is called Batesian Mimicry, where a harmless animal mimics a harmful one.
Recent investigations have shown that Viceroys are just as bad tasting as Monarchs, and that both species benefit from their similarity. A bird that eats either species will never eat either one again. This is called Mullerian mimicry, where two harmful species use the same warning colors.