What is an ER Diagram and how to draw it?

ER Diagram or Entity Relationship Diagram is a tool used to graphically represent the relational model of a database schema. It is a very important and useful tool that can be used in the Database Design phase of software development process to represent entities, relations, attributes, multiplicity, participation and constraints identified.  Various icons and notations are used to represent entities, relationships, attributes, multiplicity, participation and constraints are discussed below.