Selenium Tool Suite
Selenium is composed of multiple software tools. Each has a specific role.
Selenium 2 (aka. Selenium Webdriver)
Selenium 2 is the future direction of the project and the newest addition to the Selenium toolkit. This brand new automation tool provides all sorts of awesome features, including a more cohesive and object oriented API as well as an answer to the limitations of the old implementation.
Selenium 1 (aka. Selenium RC or Remote Control)
Selenium RC was the main Selenium project for a long time, before the WebDriver/Selenium merge brought up Selenium 2, the newest and more powerful tool.
Selenium IDE (Integrated Development Environment) is a prototyping tool for building test scripts. It is a Firefox plugin and provides an easy-to-use interface for developing automated tests. Selenium IDE has a recording feature, which records user actions as they are performed and then exports them as a reusable script in one of many programming languages that can be later executed.
Selenium-Grid allows the Selenium RC solution to scale for large test suites and for test suites that must be run in multiple environments. Selenium Grid allows you to run your tests in parallel, that is, different tests can be run at the same time on different remote machines.