In selenium webdriver,
FindElement will return you the element found using your locator.
e.g. driver.findElement(By.id("username")); //will return username text box.
FindElement will return you List of elemnets found using that locator.
e.g. driver.findElements(By.tagName("tr")); //will return you all rows in the table in form of List<WebElement>
You use findElement when you have only one element with that locator on the page. You use findElements when we want mutliple elements to be identified with the same locator type.
Hope that helps!