WebService Methods

Apex class methods can be exposed as custom SOAP Web service calls. This allows an external application to invoke an Apex Web service to perform an action in Salesforce. Use the webService keyword to define these methods. For example:

1global class MyWebService {
2    webService static Id makeContact(String lastName, Account a) {
3        Contact c = new Contact(lastName = 'Weissman', AccountId = a.Id);
4        insert c;
5        return c.id;
6    }
7}

A developer of an external application can integrate with an Apex class containing webService methods by generating a WSDL for the class. To generate a WSDL from an Apex class detail page:

In the application from Setup, enter “Apex Classes” in the Quick Find box, then select Apex Classes.
Click the name of a class that contains webService methods.
Click Generate WSDL.

Apex Developer Guide

WebService Methods