What is difference between REST and API?

I want to know the main difference between REST and API. Sometimes I see REST API in programming documents, then is REST or API same as REST API? I would like to know more about relation between REST, API and REST API.

REST is a type of API. Not all APIs are REST, but all REST services are APIs.

API is a very broad term. Generally it's how one piece of code talks to another. In web development API often refers to the way in which we retrieve information from an online service. The API documentation will give you a list of URLs, query parameters and other information on how to make a request from the API, and inform you what sort of response will be given for each query.

REST is a set of rules/standards/guidelines for how to build a web API. Since there are many ways to do so, having an agreed upon system of structuring an API saves time in making decisions when building one, and saves time in understanding how to use one.

Other popular API paradigms include SOAP and GraphQL.

Note that the above attempts to answer the question in regards to how the terms are commonly used in web development. Roman Vottner has offered a different answer below which offers good insights into the original definition of the term REST with more technical precision than I have provided here.

REST mostly just refers to using the HTTP protocol the way it was intended. Use the GET HTTP method on a URL to retrieve information, possibly in different formats based on HTTP Accept headers. Use the POST HTTP method to create new items on the server, PUT to edit existing items, DELETE to delete them. Make the API idempotent, i.e. repeating the same query with the same information should yield the same result. Structure your URLs in a hierarchical manner etc.

REST just is a guiding principle how to use URLs and the HTTP protocol to structure an API. It says nothing about return formats, which may just as well be JSON.

That is opposed to, for example, APIs that send binary or XML messages to a designated port, not using differences in HTTP methods or URLs at all.

There is no comparison in REST and API, REST is an API type.

API, in general, is a set of protocols deployed over an application software to communicate with other software components (Like browser interacting with servers) and provide an interface to services which the application software offers to several live consumers.

And Rest is a form of principle which an API follows in which the server provides information whatever the client desires to interact with services.

REST basically is a style of web architecture that governs the behavior of clients and servers. While API is a more general set of protocols and is deployed over the software to help it interact with some other software. REST is only geared towards web applications. And mostly deals with HTTP requests and responses. This makes it practically usable by any programming language and easy to test.

API is an acronym for Application Programming Interface and defines a set of structures (i.e. classes) one has to implement in order to interact with a service the API was exposed for. APIs usually expose operations that can be invoked including any required or supported arguments as well as the expected responses. Classical examples here are Corba IDL, SOAP or RMI in the Java ecosystem but also RPC-like usages of Web systems specified in documentation like Swagger or OpenAPI.

REST (REpresentational State Transfer) on the contrary was specified by Fielding in his doctoral thesis where he analyzed how the whole user interactions occurs on the Web. He realized that on the Web only a transport protocol, a naming scheme for stuff as well as a well defined exchanged format is needed to exchange messages or documents. These three parts therefore define the interface to interact with peers in such a ecosystem. The transport layer is covered by HTTP while the naming scheme is defined by URI/IRI. Contrary to traditional RPC protocols which usually only support one syntax, REST is actually independent from a particular syntax. To upkeep interoperability both client and server though need do negotiate about it, which HTTP itself supports through the Accept request and Content-Type response headers. As long as client and server support HTTP, URI/IRI and a set of negotiated representation formats, defined by backing hypermedia capable media-types, they will be able to interact with each other. In a more narrow sense REST therefore has no API other than HTTP, URI/IRI and the respective media types.

However, things are unfortunately not that easy. Most people unfortunately understand something very different in terms of REST or REST API. While URIs should not convey any semantics itself, after all they are just pointers to a resource, plenty of programmers attribute more importance to URIs than they should. Some clients i.e. will attempt to extract some knowledge off of URIs or consider URIs to return responses that represent a certain type. I.e. it may seem natural to consider an URI such as https://api.acme.org/users/1 to return a representation that describes a particular user of that particualar system. An external documentation may specify that a JSON structure is returned that follows a given template such as

{
  "id": 1,
  "firstName": "Roman",
  "lastName": "Vottner",
  "role": "Admin",
  ...
}

can be expected, however, such a thing is closer to RPC than it is to REST. Neither is the response self-descriptive, as required by REST, nor does it follow a representation format that follows a well defined media type that defines the syntax and each of the elements that may form a message. Clients therefore are usually tailor-made for exactly one particular system (or REST API if you will) and can't be used to interact with different systems out of the box without further manual integration/updates. External documentation such as OpenAPI or Swagger are used to describe the available endpoints, the payload-templates that a server will be able to process as well as the expected responses, depending on the input. These documentation therefore is the truth and thus defines the API a client can look up or even use to autogenerate stub classes to interact with the server-side, similar to SOAP.

I therefore don't agree with the answer given by dave. While for RPC systems or the common understood term of REST API his definition may be suitable, for actual REST architectures his explanation isn't fitting at all and thus, IMO at least, not correct either. REST isn't a collection of rules, standards and/or guidelines. It is a set of few constraints that just ensure that peers in such an architecture avoid coupling, support future evolution and become more robust to change.

API is basically a set of functions and procedures which allow one application to access the feature of other application

REST is a set of rules or guidelines to build a web API. It is basically an architectural style for networked applications on the web which is limited to client-server based applications.

Read more at: https://www.freelancinggig.com/blog/2018/11/02/what-is-the-difference-between-api-and-rest-api/