PayPal lets you e-mail money. With more than 15 million registered users, PayPal is probably the most popular online financial service. PayPal and its competitors let you send money by charging your credit card or transferring money from a bank account.

PayPal was a revolutionary product when it was introduced in November 1999. At first, it was marketed mostly as a way to settle small transactions with friends and family -- settling the check at a restaurant, for example.

But it turned out that using PayPal was a good way to pay for items bought from strangers in online auctions. Less than a year and a half after the first PayPal transaction, it was being used to settle more than a quarter of the auctions on eBay. PayPal is easier, faster and usually cheaper than sending a cashier's check in the mail (another favored way to pay for items bought in online auctions).

PayPal's competitors include Citicorp's c2it and its AOL Quick Cash service, Bank One's eMoneyMail and Western Union's money transfer service. For about two years, PayPal's biggest competitor was Billpoint, a payment company that was co-founded by eBay. But in the summer of 2002 eBay announced plans to buy PayPal and shutter Billpoint.

PayPal's competitors have their flaws. The other services aren't nearly as popular as PayPal, so it's easier to find someone who will accept PayPal.

If you pay with a cashier's or certified check or a postal order, you have to trek to a bank or post office to get it and you have to pay for it. Then you mail the payment and wait for the seller to receive it.

Online personal payments are easy and quick to send. The recipient doesn't have to wait for a check to clear. The payments are convenient, which is why they are popular.

PayPal lets you keep money in a stored-value account that pays interest. But it's not an insured bank account, so if PayPal were to go out of business, customers conceivably could lose their money.

PayPal and the other online payment services are constantly battling fraud. Criminals see a source of easy money. The most common form of fraud is the auction scam, in which high bidders in online auctions pay electronically, then never receive the goods. Payment services use myriad methods to fight fraud, from verifying customers' credit cards to confirming that users are who they say they are by authenticating their bank accounts.

Every payment service has its own privacy policy. PayPal says it doesn't share personally identifiable information with outsiders except to verify credit cards.