What is asmx concise of???

An abbreviation for Active Server Method File, a file with the ASMX file extension is an ASP.NET Web Service Source file.ASMX provides the ability to build web services that send messages using the Simple Object Access Protocol (SOAP). SOAP is a platform-independent and language-independent protocol for building and accessing web services.

This is what MSDOCs says

ASMX provides the ability to build web services that send messages using the Simple Object Access Protocol (SOAP). SOAP is a platform-independent and language-independent protocol for building and accessing web services. Consumers of an ASMX service do not need to know anything about the platform, object model, or programming language used to implement the service. They only need to understand how to send and receive SOAP messages. 

Hi,

ASMX file is a web service and it is XML Services based on SOAP.

Wikipedia web service definition - The term Web services describes a standardized way of integrating Web-based applications using the XML, SOAP, WSDL and UDDI open standards over an Internet protocol backbone. XML is used to tag the data, SOAP is used to transfer the data, WSDL is used for describing the services available and UDDI is used for listing what services are available. Used primarily as a means for businesses to communicate with each other and with clients, Web services allow organizations to communicate data without intimate knowledge of each other's IT systems behind the firewall.

Reference :

https://forums.asp.net/t/964152.aspx?Need+Concise+Definitions+for+aspx+and+asmx