ER Diagram is an abstract and conceptual representation of data. Entity-relationship modeling is a database modeling method, used to produce a type of conceptual schema or semantic data model of a system, often a relational database, and its requirements in a top-down fashion. Diagrams created by this process are called entity-relationship diagrams, ER diagrams, or ERDs (wikipedia).  How to Draw ER Diagrams is quiet simple, when I was surfing on youtube I found this great tutorial, watch carefully video tutorial bellow "How to Draw an ER Diagram with DIA" .

Besides of  Drawing ER Diagram, you can do following action with DIA : 
  • Creating Flowchart Diagram.
  • An UML Diagram
  • Network Diagram
  • or Something like a Site plan.