Most people know eBay Inc. for its online marketplace, where deals abound on everything from gadgets to antique furniture. But soon, eBay's biggest business will likely be PayPal, the online payments service that has been growing steadily even as the economy has stumbled.

EBay has spent much of the past two years trying to improve its faltering marketplace business, hoping to increase buyers' trust and clean up the look of its website. In the meantime, PayPal has thrived as more consumers and merchants use it to send money online.

Its growth is expected to continue in spite of competition from Amazon.com Inc. and Google Inc., which have services that online retailers sometimes offer alongside PayPal.

PayPal bills itself as a shopper's online wallet. Users set up accounts and link them to bank accounts and credit cards, making it easy to transfer cash into the account. Then users can make payments through PayPal using either their cash balances or the underlying credit card. PayPal users can also send cash to someone based on as little information as an e-mail address or cell phone number.

But unlike what happens with a credit or debit card online, PayPal doesn't share your financial information with merchants.

PayPal, which began in 1998 as a way for people to beam cash from one Palm Pilot to another, was bought by eBay for $1.5 billion in 2002 and has been a steady performer. The service charges fees for certain transactions, and in the most recent quarter it reported $688 million in revenue, a 15 percent jump from last year. As of the end of September, 78 million people had active PayPal accounts, up from 65 million a year ago.

To try to maintain its advantage, this week PayPal opened its system to third-party developers, which will mean PayPal can be built in to all sorts of applications. For instance, an iPhone app could let consumers order a pizza and pay for it with PayPal.

The company projects PayPal's revenue will be between $4 billion and $5 billion in 2011.