This is text-based API, develop for variety of reason, such as to create a tool to facilitate internal processes or an external product for customers or to build a third party tool.

This is the event-driven, serial-access mechanism that does element-by-element processing. This type of API reads and writes XML to a data repository or the web.

That breaks down a transaction into a series of small modules, which addresses a particular underlying part of the transaction, and server don't maintain state of previous interactions.

SOAP communicate between applications running on varies OS, technologies & programming languages over HTTP, because HTTP is supported by all browsers and servers.