An Apex callout enables you to tightly integrate your Apex with
an external service by making a call to an external Web service or sending a HTTP
request from Apex code and then receiving the response. Apex
provides integration with Web services that utilize SOAP and WSDL, or HTTP services (RESTful
services).
Before any Apex callout can call an external site, that site
must be registered in the Remote Site Settings page, or the callout fails. Salesforce prevents calls to unauthorized network addresses.
If the callout specifies a named
credential as the endpoint, you don’t need to configure remote site
settings.
A named credential specifies the URL of a callout
endpoint and its required authentication parameters in one definition.
To set up named credentials, see “Define a Named Credential” in the Salesforce Help.
To learn more about the types of callouts, see:
Callouts enable Apex to invoke external web or HTTP services. Apex Web services allow an external application to invoke Apex methods through Web
services.