You may have gotten confused because most resources that describe SOAP-based integration usually refer to using "callouts", "web services", or something dealing with "WSDL", but rarely just "SOAP."
For example, if you create an Apex Code class that can be called by the SOAP API, the corresponding command to call the integration will generate a WSDL file you'll import into your IDE. Likewise, when you want to call a SOAP-based integration, you'll get a WSDL from that system and import it into salesforce.com.
Calling Salesforce Web Services Using Apex gives an example of a Salesforce-to-Salesforce WSDL-based (and thus, SOAP-based) integration, and Apex Web Services and Callouts shows how to set up a web services class using "WSDL2Apex".
There are many more resources, both official and unofficial. I recommend doing a search on your favorite search engine for "salesforce web services" for plenty of documents that describe building and consuming web services into and out of salesforce.