PDF Tools is, among other things, a replacement of DocView for PDF files.
The key difference is that pages are not pre-rendered by e.g. ghostscript
and stored in the file-system, but rather created on-demand and stored in
memory. This rendering is performed by a special library named, for
whatever reason, poppler, running inside a server program. This program is
called epdfinfo and its job is to successively read requests from Emacs and
produce the proper results, i.e. the PNG image of a PDF page. Actually,
displaying PDF files is just one part of pdf-tools. Since poppler can
provide us with all kinds of information about a document and is also able
to modify it, there is a lot more we can do with it.