findElement(): is used to find the first element in the current web page matching to the specified locator value. Take a note that only first matching element would be fetched. This is for single element only.
findElements(): is used to find all the elements in the current web page matching to the specified locator value. Take a note that all the matching elements would be fetched and stored in the list of WebElements. This is for multiple elements.
Example
WebElement element = driver.findElements(By.xpath(""));
List <WebElement> elementList = driver.findElements(By.xpath(""));