REST-based (or RESTful) web services are collections of web resources. In terms of REST, objects are understood as resources that are uniquely identified via an URI (Unified Resource Identifier) and represented in different formats (such as text, image, audio, video, application). Communication is established via HTTP methods (most common are GET, POST, PUT, DELETE). By implementing these common HTTP methods a CRUD (Create Read Update Delete) interface is created by the RESTful service. Therefore, no further protocol layer, such as SOAP, is necessary. For communication messages different formats can be used. A quite common choice is JSON (JavaScript Object Notation).

In the context of web services, this means that a component which implements a web service that uses a REST binding (a so-called RESTful web service) is to be understood as follows:

REST services are the most common web service type. Almost all large companies and projects provide REST APIs, such as Flickr, Google Maps and Amazon. Apache NetBeans IDE Software as a Service (SaaS) functionality lets you use Facebook, Zillow, and other third-party-provided services in your own applications.

SOAP is a messaging protocol for web services (formerly known as Simple Object Access Protocol). Nowadays SOAP also serves as a synonym for web services that rely on SOAP. SOAP incorporates and relies on a lot of so called WS* standards. SOAP itself includes definitions for calls of remote services (methods), message exchange and message structures (especially their packaging, called envelope and their encoding). SOAP relies a lot on XML, hence definitions and messages are typically created as XML documents.

The interface description of a SOAP web service is also provided in XML, by a so called WSDL (Web Services Description Language) document. WSDL is actually a standard of its own. The creation of SOAP web services can follow contract first or code first principles. In a contract first scenario a WSDL document is used to generate service stubs. In a code first scenario a component is declared to expose a SOAP web service and a WSDL document is generated from this declaration.

The WSDL is exposed on the net to make the service accessable. Parties interested in using the web service can create a client based on the WSDL. The WSDL file also defines possible operations for a given service endpoint and hence, the range of operations to a single endpoint (URI) can be much broader than what is available in REST (HTTP methods). Also SOAP services may use transport protocols other than HTTP.

Currently, most services are implemented following the REST paradigm. SOAP is still being in use. While SOAP can be convenient in terms of development, it also has some burden, primarily in terms of performance and in standardized (CRUD) interfaces. A comparison between concepts in SOAP and REST is shown in Comparison between REST and SOAP.

Also there is another paradigm called GraphQL, it was introduced by Facebook in 2012, made open source in 2015 and is currently maintained by the GraphQL Foundation. GraphQL is said to be a query language for APIs. Similar to REST, there are discussions about whether GraphQL is a web services approach.

GraphQL is becoming increasingly popular and well-known APIs are now also offered via GraphQL (for example, the GitHub API is now available not only as a REST API, but also as a GraphQL API). It remains to be seen whether GraphQL will replace REST in the future. Although there are Java libraries that enable GraphQL, there is currently no Java standard for GraphQL.