A DISCO file is an XML document that contains the URLs of WSDL (Web Service Description Files) .
It enables programmatic discovery of the location of web services available at a web server. By pointing to the location of WSDL files, programmers can interrogate the WSDL document and discover how to invoke a web service.
E.g. a disco file could point to a .asmx and WSDL files using the URL : http://www.visual-basic-data-mining.net/dataaccess.asmx?WSDL , the DISCO file itself could be located at http://www.visual-basic-data-mining.net/Default.DISCO
The DISCO process differs from UDDI (Universal Description, Discovery and Integration) because UDDI is essentially a public or private registry where information about available DISCO documents can be located, while the DISCO documents when located point to WSDL documents.
References :
WSDL (http://www.visual-basic-data-mining.net/Forum/ShowPost.aspx?PostID=329)