Magento choose to use some interaction client as service (click) , for example when you try to place order you get url like rest//V1/guest-carts/:cartId/payment-information , ok but how you can see where is point this url ? Is a module controller action url ? Wrong is a service .
Go and open the webapi.xml inside folder etc Yeah : you will find all the service related to current module . For example :

 
    <!-- Guest place order with payment information saving -->
    <route url="/V1/guest-carts/:cartId/payment-information" method="POST">
        <service class="Magento\Checkout\Api\GuestPaymentInformationManagementInterface" method="savePaymentInformationAndPlaceOrder"/>
        <resources>
            <resource ref="anonymous" />
        </resources>
    </route>
 

But how you can read :
Easy you will use as post(method) ,  is related to interface  GuestPaymentInformationManagementInterface , inside it you can find all abstraction of service , but the real class to execute is Magento\Checkout\Model\GuestPaymentInformationManagement magento use preference to create relation between service and model :

 
    <preference for="Magento\Checkout\Api\GuestPaymentInformationManagementInterface" type="Magento\Checkout\Model\GuestPaymentInformationManagement" />
 

Now is the time to explore the method  savePaymentInformationAndPlaceOrder inside 
 Magento\Checkout\Model\GuestPaymentInformationManagement .

You can create you're own service by using this references :
https://alankent.me/2015/07/24/creating-a-new-rest-web-service-in-magento-2/ 
http://vinaikopp.com/2017/02/18/magento2_repositories_interfaces_and_webapi/ 
http://inchoo.net/magento/api-magento/magento-2-custom-api/