Go to yourdon.com for a great Wiki source for Data Flow Diagrams (DFDs).
Ed Yourdon and Tom DeMarco wrote important books in the 1970's. They're work on structured programming and analysis layed the groundwork for later efforts in object-oriented programming and analysis.
These days, I feel that their work has a new relevance in the web service and business process arena. UML is intimidating. E-R diagrams are of limited use in discussing information flow (or how two IT projects fit together). The simplicity of the symbols is very powerful and I've found that even non-technical people can understand them.
Components of a DFD