First, they are based on the XML specification. Many modern applications already speak XML, and it's usually not hard to wrap XML around older systems or applications using APIs or terminal emulation. Third, they are highly programmable and extensible. HTML provides end users with information on demand. XML offers access to well-defined data.
Web services enable the remote execution of application components.
Enterprise portals take this concept to the next level. Although companies like Microsoft and BEA Systems are moving full throttle onto the Web services racetrack, bear in mind that the technology is just in its earliest stages. Many details of the technologies and business models behind Web services must be worked out before you deploy them for more than pilot projects. Take high availability, for example. Perhaps the vendor went out of business.