Does Marketo use a SOAP interface to 'talk' with an outside App? If not, what interface does it use?
Marketo provides multiple API's to "talk" with other apps
In both case, it will take a developper to develop the connection to these API's: these are inbound API's, meaning that the other app will have to connect to Marketo and manage the transactions.
There are also some client side API's (that enable to connect to Marketo from a visitor's browser).
The question's a bit vague... Marketo supports inbound SOAP (though that API is deprecated and shouldn't be used for new projects). Marketo's outbound API is webhooks, which traditionally use either form-like/URL-encoded or REST-like/JSON communications but could use SOAP. An intermediate webhook can also be used as a gateway to a (2nd-hop) remote SOAP service.
You should elaborate on your requirements to get a more precise answer.