Let's try to create an example where you need to find an element using the CSS selector. This is used by findElementByCssSelector and findElementsByCssSelector.
For example,
using indElementByCssSelector:
Driver.get("http://book.theautomatedtester.co.uk")
WebElement element = ((FindsByCssSelector)driver).findElementByCssSelector("Chapter1");using findElementsByCssSelector:
driver.get("http://book.theautomatedtester.co.uk")
List<WebElement> elements = ((FindsByCssSelector)driver).findElementsByCssSelector("Chapter1");
Assert.equals(1, elements.size());