A level 0 data flow diagram (DFD)
Known as a context diagram, shows a data system as a whole and emphasizes the way it interacts with external entities.

A level 1 data flow diagram (DFD)
A more detailed than a level 0 DFD but not as detailed as a level 2 DFD. It breaks down the main processes into sub-processes that can then be analyzed and improved on a more intimate level.

Level 2 data flow diagram (DFD)
This offers a more detailed look at the processes that make up an information system than a level 1 DFD does. It can be used to plan or record the specific makeup of a system.