High level architecture of the SeleniumLibrary
The test data contains the actual test cases and possible resource files needed by them. These files must be in format understood by Robot Framework.
Robot Framework is a generic test automation framework that uses different test libraries to interact with the applications under test.
SeleniumLibrary, as you probably know if you are reading this documentation, is a web testing library for Robot Framework. As the name implies, it uses popular Selenium web testing tool internally. When the library is used for Flex testing it uses also Flex Pilot tool.
The SeleniumLibary uses Selenium Remote Controller's (RC) Python client library to communicate with the Selenium Server.
The Selenium Server is the core component of the Selenium Remote Controller. It is responsible on launching browsers and interacting with them. It is a Java application, but there are client libraries for various programming languages. Using these bindings requires that the Selenium Server is running.
Probably the best feature of the Selenium tool is that it supports various different browsers. See the Open Browser keyword for more information on how to use different browsers with the SeleniumLibrary and Selenium documentation for a full list of supported browsers.
To easily run tests with different browsers, it is generally a good idea to specify the browser that the Open Browser keyword with a variable in the test data. This functionality is demonstrated well by the SeleniumLibrary demo.
The most important component is obviously the web application you are testing.