What are Call outs in salesforce?What is a Callout method? How does it invoke, how many methods available in Classes and Triggers?

Callouts are mainly used to invoke the External services HTTP or web services.

In simple words if we want to connect to external system we need to use callouts.

Apex callout enables you to integrate your Apex with an external service by making a call to an external Web service or sending a HTTP request from an Apex script and then receiving the response.

Apex provides integration with Web services that utilize SOAP and WSDL, or HTTP services (RESTful services).