Pbuilder constructs a chroot system, and builds a package inside the chroot. It is an ideal system to use to check that a package has correct build-dependencies. It uses apt extensively, and a local mirror, or a fast connection to a Debian mirror is ideal, but not necessary.
"pbuilder create" uses debootstrap to create a chroot image.
"pbuilder update" updates the image to the current state of testing/unstable/whatever
"pbuilder build" takes a *.dsc file and builds a binary in the chroot image.
pdebuild is a wrapper for Debian Developers, to allow running pbuilder just like "debuild", as a normal user.