SOAP
In plain terms
A rigid, XML-based protocol for structured web services.
Going deeper
SOAP exchanges strongly-typed XML messages with strict contracts (WSDL) and built-in standards for security and transactions. It predates REST and remains in enterprise systems.
Under the hood
Verbose and heavyweight but offers formal contracts and WS-* features; largely supplanted by REST/gRPC.
Examples
Legacy enterprise integrations, banking, government services, WSDL APIs.
Think of it like...
Like formal certified mail with strict forms, versus a quick postcard (REST).