What they do is have a hidden paragraph buried in the very lengthly legal terms that you agree to when you click the "I accept" button. This is the one that no one reads or it would take a whole day. What it says is that you automatically agree to renew when your subscription ends.
For instance, say you sign up for a three month membership. When the three months ends, you think your membership is over and your credit card won't be charged again, right? Wrong! Match.com rips you off by continuing to bill your credit card until you call them and complain, all because of this buried paragraph designed by their shyster lawyers.
My advice is to stay away from Match.com; there other other dating websites that don't screw you like this. Match.com has a bad reputation for ripping people off.