What is Selenium WebDriver and how is it different from Selenium IDE?

Selenium IDE = record-and-playback browser extension, great for demos and quick throwaways. Selenium WebDriver = the actual automation library that speaks the W3C WebDriver protocol from your code (Java/Python/JS/C#/Ruby) to a browser driver (chromedriver, geckodriver). Real automation projects use WebDriver; IDE is a learning/demo tool.