Most credit cards are insured against fraud, by credit card insurance policies held by credit card companies. You'll either pay nothing if someone uses your card, or you'll pay a very small fee. All you have to do is keep your eye on your statements. If you didn't personally purchase the item, most likely, you won't have to pay for it.
This isn't the first "is it safe to use credit cards?" thread. I would be interested to hear from someone that had been defrauded, and the credit card company didn't reimburse them. Credit card companies pay for credit card insurance, but they gain consumer confidence, which ultimately pays off in greater spending, and greater revenue from interest rates.
Using credit cards on the net is certainly far safer than handing your card over to the high-school kid at the gas station. However, "Hacker steals 1 million credit card numbers" makes for a great TV news headline.
In addition to fraud protection, you have other added benefits.
PayPal is similarly convenient. However, they do charge a fee that may be higher than what you're paying with your credit card. And, they typically provide less rights than you have when using your credit card. More info at http://www.paypalsucks.com