Selenium is a testing tool and comprises of mostly three components.
- Selenium IDE: It is a tool for recording and playing back. It is a Firefox plugin.
- WebDriver and RC: It provides the APIs for a variety of languages like Java, .NET, PHP, etc. They work with most of the browsers.
- Grid: you can distribute tests on multiple machines so that test can be run parallel which helps cutting down the time required for running test suites in the browser.
Hope this was a good brief description for what you were looking for.