The Chinese Astrology and its 12 signs Zodiac has a very long and interesting history, surprisingly enough, the 12 animal signs we know today are a relative recent invention, not much older than 1000 years, and not just that, but is also thought that they weren't a chinese invention but that they could have been imported into China from Mongolia or even Siberia.
One funny thing about Chinese Astrology is that when you study it in deep, you find that is full of mysteries and strange occurrences!
Maybe the most common source of confusion about the Chinese Zodiac arises from images like that above, you see that it relates each animal sign with one western year, don't you? Well, actually it doesn't work that way, most serious astrologers use either the February 4th to mark the beginning of the chinese astrological year, or either the date of the chinese lunar new year (this is when the chinese make those big parties with dancing dragons on the streets).
But well, the truth is that the actual 12 signs of the Chinese Zodiac somewhat work, I mean that when you look close at it, see what signs are the people you know and which are their supposed characteristics, in the whole it makes much more sense than most people would expect.
You don't need to believe me, just give it a try and see for yourself ;-)