Selenium, as of today, is composed of several projects :
Selenium IDE The Firefox plugin used to build test-cases and test-suites directly on top of your web-app. You can also export your tests to Selenium RC.
Selenium RC A client/server system that allows you to execute tests written in a variety of languages on a local or remote computer and on all major browsers.
Selenium Grid Selenium Grid leverages Selenium RC to provide a test environment that spans on multiple machines at once, reducing the testing time.
Selenium on Rails A framework that is built especially to test applications running on the Rails framework.
CubicTest An Eclipse plugin which purpose is to conceive and execute tests using the pageObjects pattern.
Bromine A simple web-based QA tool targeting the single tester that doesn’t want/can’t invest in complex corporate QA solutions.
While being very popular, Selenium is not the ultimate all-in-one testing tool and you should NOT rely on Selenium only to test your applications. The following chart helps you situate Selenium among the web-testing stack.
Multi-controller tests : htmlunit, webrat, cucumber
View testing in isolation : rspec, Selenium (see Section 9.1.2, “Unit testing”)
Functional Testing/Controller testing
Unit testing/Model Testing : server side, depends on backend technology