The integration between 2 application/platform to perform the action of integration is termed as API (Application programming interface).
Integration will be in the forms of URL that contains service address with hosted location (server).
REST and SOAP are the 2 API services.
The REST and SOAP has its own unique properties and behaviour.
SOAP will support only XML structure of data format.
REST supports all the structure of data format viz JSON.
WSDL (Web Services Description Language) based call services are SOAP API.
AX 2012 cannot be integrated directly from any API, for that we have to create serviceReference in Visual Studio and to be deploy the project in AOT to consume the functions and object present in that services.
AX2012 supports only SOAP web services.
D365 can be integrated by custom web services or ODATA. It creates 2 endpoints one for REST and other of SOAP.