There are three different styles of services:
- RPC API - the client sends a procedure and parameters to service and the service is responsible for the executing of the command and returning a result.
- Message API (Document API) - the client sends DOMs (elements), which normally are more complex structures than RPC API calls, because they tend to do not imply operations directly.
- Resource API - is used for accessing resources (database tuples, files, images and etc.). In general it should also provide good Media Type Negotiation.
SOAP and REST are compilation of standards from W3C, and the main difference is that SOAP uses HTTP, SMTP and etc. as transport protocols and REST uses it as application protocol, AKA it should support (GET, PUT, PUSH, DELETE, and POST). SOAP also implies using XML and REST could use any data type (JSON, XML, HTTP, etc.). Furthermore, one of the main advantages of SOAP is the Service Descriptor (WSDL file), which gives the possibility of auto-generation of Service Connector (proxy) to the client.
There is not a silver bullet; the type and architecture of a web service are dependent on the actual client and technology requirements.
For a general idea on the subject, see one of the Martin Fowler signature books - Service Design Patterns
