To the naked eye, there's not that much difference between online services and the Web anymore. But the differences used to be much more marked.

Customers used to pay the four big online services--America Online (AOL), CompuServe, Prodigy, and the Microsoft Network (MSN)--to get access to worldwide networks with libraries of digital information, to send email around the world, and to join online communities where people with similar interests could communicate for business or pleasure.

The online services offered all this by building and maintaining long stretches of interconnected wires and servers that no one but their customers could use. Each system had a different interface and customers nearly always paid by the hour or the minute. The companies competed over which one was easiest to use and which one had the best content.

But then the public noticed the Web, which offered a vast network with an interface--the browser--that was the same for everybody. Where the online services relied on vast private networks, the Web wasn't owned by anyone in particular. To get on America Online's network, you had to pay them. To get on the Web, you had to pay any regional or national ISP for a local dial-up number. Suddenly, the online services were competing with hundreds of ISPs, instead of only each other.

So the online services adapted: they started letting users roam the Web as well as their own networks. And they got cheaper, substituting monthly flat fees for per-hour surcharges. To most users, online services are now simply big ISPs. Prodigy has even taken to calling itself "the SuperISP."

Of the big four, only one--AOL--is still determined to maintain its own private network. MSN, Prodigy, and CompuServe have all instead constructed huge Web sites to serve as new homes to all of their exclusive content. They still want you to pay, but for access to the Web and to content you can't see anywhere else instead of as an entrance fee for their private networks.

CompuServe and Prodigy are using the wires they still control to sell "premium" Internet services--that is, guaranteed Net service to businesses that demand total reliability. MSN, on the other hand, doesn't care which ISP you use: it is more interested that you sign up to get at its content.

There still seems to be a market for private networks among users who have never used the Net before; this is where most of AOL's new customers come from. Disney is also launching a new online service aimed at children and families. But the distinction between subscribing to a private network like Disney's Daily Blast and subscribing to a private Web site like MSN is becoming more and more blurred.