What's the difference? I know that
\pagebreak can take an optional argument that specifies how much a page break is desired, but is there any difference if the optional argument is not used?
Both commands start a new page.
If the optional argument is not used,
\pagebreak will start a new page and the paragraphs of the old page will be spread out so that the old page will not look like the end of a chapter.
\newpage, on the other hand, the old page will have the blank space at the bottom, because the paragraphs will stick together as if the chapter had ended there.