Selenium Overview

Selenium is a suite of tools to automate web app testing across many platforms. It is a GUI based automation tool. Initially it is built by ThoughtWorksThoughtWorks. It supports various browsers on various platforms
Selenium Projects
Selenium has many projects. Following projects are mostly used by testers.
1.Selenium IDE (IDE)
Selenium IDE can be used only in FireFox. It is an add-on for FireFox. User can record the actions and can edit and debug the tests. It can be used to identify IDs, name and XPath of objects. Only one test at a time.
2.Selenium Core (CORE)
Selenium Core is the original Javascript-based testing system. This technique should work with any JavaScript enabled browser. It is the engine of both, Selenium IDE and Selenium RC (driven mode), but it also can be deployed on the desired application server. It is used specifically for the acceptance testing.
User can record the tests using Selenium IDE and can use the same tests to run in other browsers with minimal modifications. It provides support to run the tests in HTA (HTML Applications) Mode. This mode works only in IE.
3.Selenium Remote Control (RC)
Selenium Remote Control is a test tool that allows user to write automated web application UI tests in few programming languages against any HTTP website using any mainstream JavaScript-enabled browser. User can write the tests (More expressive programming language than the Selenese HTML table format) in Java, DotNet, Perl, Ruby and PHP. Also it supports few testing frameworks.
4.Selenium Grid
Selenium Grid allows easily to run multiple tests in parallel, on multiple machines, in an heterogeneous environment by cutting down the time requirements