Here is a quick way to capture a screenshot and save it into a file:
Screenshots API accepts many parameters for image format, clip area, quality, etc. Make sure to check them out.
Full page screenshot is a screenshot of a full scrollable page, as if you had a very tall screen and the page could fit it entirely.
Rather than writing into a file, you can get a buffer with the image and post-process it or pass it to a third party pixel diff facility.
Sometimes it is useful to take a screenshot of a single element.