DFD's are simple diagrams that deal with 3 things:
Processes (Discrete programs, functions or manual processes)
Flows (Chunks of data that move from one process to another)
Stores (Where data is stored or retrieved from)
Processes are circles, rectangels or elipses
Flows are lines with a name
Stores are parallel lines, or thin rectangles or elipses
For a website, your functions typically will be one page. Your flows will be data that goes between pages, that needs to be passed as url params, or in Posts, or stored in session variables.
Often however, website designers use sitemaps to do high level website design.
I've lnked to one of the seminal books about traditional analysis diagrams and process, which is completely online in a wiki format. That book is Ed Yourdon's Structured Analysis.
I've linked you to the section on DFD's.