QUICK TOUR
Selenium is not just one tool or API but it composes many tools.
WEBDRIVER
WebDriver is also known as Selenium 2. If you are beginning with desktop website test automation then you are going to be using WebDriver APIs. WebDriver uses browser automation APIs provided by browser vendors to control browser and run tests. This is as if a real user is operating the browser. Since WebDriver does not require its API to be compiled with application code, it is not intrusive in nature. Hence, you are testing the same application which you push live.
SELENIUM REMOTE CONTROL
Selenium Remote Control is also known as Selenium 1. Selenium RC was the most prominent Selenium tool before the advent of Selenium WebDriver. Selenium RC would use a proxy server and inject JavaScript into a browser to be able to control it. Given the intrusive nature Selenium RC had on a browser, you could never be sure if what you were testing was the same as the application you wanted to push live. Selenium 2 APIs yet contain Selenium RC APIs but Selenium 3 would completely get rid of Selenium RC APIs. If you are still using Selenium RC then you must migrate to Selenium WebDriver.
SELENIUM IDE
Selenium IDE is a Firefox plugin which can be used to record test steps in Firefox itself. Selenium IDE can be used to generate quick and dirty test code in a variety of programming languages (i.e. C#, Java, Python, and Ruby). Given the maintainability of code generated through Selenium IDE, it is not recommended to use it for anything more than getting acquainted with element locators or generatingthrow away code. We're sure that once you get used to the WebDriver API, you will never use Selenium IDE.
SELENIUM GRID
Soon after development of WebDriver tests, you may face a need of running your tests on multiple browser and operating system combinations. This is where Selenium Grid comes to the rescue.
SELENIUM HTML RUNNER
This tool allows you to run Test Suites from the command line. Test Suites are HTML exports from Selenium IDE or campatible tools. Selenium HTML-runner
快速导览
Selenium不仅仅是一种工具或API，它由许多工具构成。
WEBDRIVER（掌握）
WebDriver也就是Selenium 2.如果您从桌面网站测试自动化开始，那么您将使用到WebDriver APIs。WebDriver使用浏览器供应商提供的浏览器自动化APIs来控制浏览器并运行测试。这就好像一个真实用户操作浏览器。由于WebDriver不需要使用应用程序代码编译其API，因此它本质上不具侵入性。因此，您正在测试的应用程序与您推送的应用程序相同。
Selenium 远程控制器（废弃）
Selenium Remote Control也被称为Selenium 1.Selenium RC是Selenium WebDriver出现之前最重要的Selenium工具。Selenium RC将使用代理服务器并将JavaScript注入浏览器以便能够控制它。鉴于Selenium RC在浏览器中的侵入性，您无法确定您测试的内容是否与您希望推送的应用程序相同。Selenium 2 APIs包含Selenium RC API，但Selenium 3将完全摆脱Selenium RC APIs。如果您仍在使用Selenium RC，那么您必须 迁移到Selenium WebDriver。
Selenium IDE
Selenium IDE是一个Firefox插件，可用于记录操作Firefox的测试步骤。Selenium IDE可用于以各种编程语言（即C＃，Java，Python和Ruby）生成快速且粗糙的测试代码。鉴于通过Selenium IDE生成的代码的可维护性，建议不要将其用于除了了解元素定位器或生成一次性代码之外的任何事情 。我们确信，一旦您习惯了WebDriver API，您将永远不会使用Selenium IDE。
SELENIUM网格
在WebDriver测试开发后不久，您可能需要在多个浏览器和操作系统组合上运行测试。这是Selenium网格 应对的地方。
SELENIUM HTML RUNNER
该工具允许您从命令行运行测试套件。测试套件是Selenium IDE或兼容工具的HTML导出。Selenium HTML-runner