The information moves from one place to another.
When it is moved, certain actions are performed over it.
This movement is controlled by a set of rules.
These are documented easily through the state transition diagram.

Trouble Ticket System

Tabular Form

External Inputs

These are elementary processes come from outside the system to process or control information and may alter the system. It's primary intent is to maintain ILFs (Internal Logic Files).

External Outputs

These are the elementary processes which send information outside of the application boundary.
Their primary intend is to present the information to the user.
The processing logic must contain at least one mathematical structure or should create derived data.
They can also maintain ILFs.

External Inquiry

It also sends data outside the application boundary and presents data to the user derived from the ILFs.
It doesn't contain any mathematical formulas.

Data Flow Model

DFD Notation

Process: Rectangle
External Entity: Rectangle
Data store:

Data Flow: Arrow

Flow Charts

These are sequential.
Usually would be used to represent algorithms.

Example: