Web Services as we know are of two types

1. SOAP Web Service

 Here for examples we use WSDL as form of communication between Server and Client.
 Data trasferred between server and client as xml data.

2. REST Web Service

Here there is no WSDL Concept. 
Data transferred between server and client as json data.

Both makes us communicate when technologies of server and client differ or may be same.
Nowdays REST Web Service is being commonly used. 

It is much faster compared to SOAP in following ways.

a) Everytime we add a web method , we need to take care of wsdl which need to be send to client.
b) We tend to make mistake in generating wsdl.
c) Android developers face lot of difficulty in using SOAP Web Service.
d) With REST Web Service and its json support , makes data trasfer from android to Server easy and less time consuming.
e) Just need to create POJO classes for data transfer. 
f) Good use of Annotations , makes developing server side code easy and at a much faster pace.