Astrology is the following and interpreting of the planets and their influences on us.
I don't follow astrology, at least not closely. I DO believe that things outside our understanding have influence on us, but I do not believe that the stars and planets' influence is really what causes it, nor do I think that it is worth it to learn such a complex art as astrology is. My horoscopes have never been all that accurate and I don't have many of the attributes of a Capricorn or of the sign I'm mighty close to, Aquarius. I have about as much of those two as I do all the others. However, I do think that astrology is somewhat useful in practicing magick, in that you just begin to realize that you are affected by a lot of things you don't necessarily understand. I do follow the Moon's movement because that is so close to my own cycle. Supposedly you are not supposed to work any magick when the Moon is "void of course," which means it's not in any of the astrological houses. This happens a lot, but not for long periods of time. Despite the fact that I don't totally believe that if the Moon wasn't in an astrological house it would throw everything off, some people like to play it safe by making sure!
In any case, I like Tarot cards and astrology because their symbolism is neat, and it's useful in representing things in spellwork and other magickal stuff . . . despite the fact that I put little stock in it, it's very interesting to me and I have fun learning about it. The Fire signs are Aries, Leo, and Sagittarius. The Air signs are Gemini, Libra, and Aquarius. The Water signs are Cancer, Scorpio, and Pisces. The Earth signs are Taurus, Virgo, and Capricorn. This is just for your information in case you are interested in knowing which element you are supposedly aligned with. You can put interesting things together with this. Like, if you have a friend who is a Gemini and you are a Virgo, and you wish to send the person strength, you might make a small wand with symbols or runes that relate to the person, bury it in some earth, and sprinkle the top with bay leaves. This breaks down like this: If the person is a Gemini, he is an Air sign, which aligns him with the element of Air and therefore the tool of the wand. Making a symbolic wand of your friend, burying him in soil (symbolic of your sign if you are a Virgo, since you are aligned with the Earth), and covering it with bay leaves (symbolic of strength) is like a symbolic way to surround your friend with your strength. Fun, eh? You can come up with things like this too. It is just interesting to know what everything stands for so that you have a wide variety of choices as to how to work your magick.
If you want more information on astrology, try Astrology's Element Article and astrology.com.