WebDriver is an API for controlling web browsers, imitating a real user. It's mostly used for automated tests. WebDriver has multiple language bindings and drivers (allowing to control various browsers). If your question is specific to one of them, make sure to also include the appropriate tag.
WebDriver is an API that allows programs or scripts to introspect into, and control the behavior of, a web browser. It is primarily intended for allowing tests that automate a browser.
The WebDriver API is implemented, natively or via add-ons, for many browsers. The Selenium tools suite provides several WebDriver implementations, e.g. for Firefox or Internet Explorer. Other WebDriver implementations are developed by the browser projects directly, e.g. ChromeDriver for Chrome, or Ghost Driver for PhantomJS.
The WebDriver API specifies a wire protocol which is used for the communication between a WebDriver client and the browser. Clients do not speak the wire protocol directly, they use "language bindings" of the WebDriver API for the respective programming language. The wire protocol is used by the Selenium Grid to pass API commands across network.
There are language bindings of the WebDriver API for Java, C#, Python, Ruby, Perl, PHP, JavaScript, and other languages. Many of the language bindings are provided by the Selenium project. For some languages, there are even multiple language bindings, e.g. WebDriverJs by Selenium and WebdriverJS by webdriver.io.
