what's the diferrent between "100px" and "100%"

Answer 1

100% all the page..
but 100px only 100 from the all size..

Answer 2

100% doesn't necessarily mean the entire page. Take <img> for example. 100% would mean that the image would show on the page with the height and width of the source image; it wouldn't be scaled at all. 100px literally means you'd be scaling the image up/down to 100px, be it width or height.

Answer 3

"100%" will be in the size of the page