WebDriver is an interface, while ChromeDriver is a class which implements WebDriver interface. Actually ChromeDriver extends RemoteWebDriver which implements WebDriver. Just to add Every WebDriver like ChromeDriver, FirefoxDriver, EdgeDriver are supposed to implement WebDriver.
Below are the signatures of ChromeDriver and RemoteDriver classes
public class ChromeDriver extends RemoteWebDriver
implements LocationContext, WebStorage {}
public class RemoteWebDriver implements WebDriver, JavascriptExecutor,
FindsById, FindsByClassName, FindsByLinkText, FindsByName,
FindsByCssSelector, FindsByTagName, FindsByXPath,
HasInputDevices, HasCapabilities, TakesScreenshot {}