Selenium is a suite of tools to automate web browsers across many platforms. WebDriver, part of that suite, is a web automation framework that allows you to execute your tests against different browsers.
Web testing is mission critical for big business and it's becoming more complicated with each passing day. With WebDriver you can create robust, browser-based regression automation suites and tests scale and distribute scripts across many environments. It also enables you to use a programming language in creating your test scripts and is the core technology in many browser automation tools, APIs and frameworks.
Selenium WebDriver 3: Level 1 Fundamentals of Automation Testing
Selenium WebDriver 3: Level 2 Implementing Test Automation Frameworks