Entity Relation Diagram which is also known as an E-R diagram is a data relation diagram.  The entity relation diagram uses specialized graphical symbols for illustrating all of the interrelationships between entities and attributes in the database.   It represents the arrangement and relationship of data entities for the logical data structure. 

There are three general graphical symbols used in entity relation diagram and these symbols are: box, diamond and oval.  The box is commonly used for representing the entities in the database. The diamond is typically used for representing the relationships and finally, the oval is used for representing all the attributes.

In many other entity relation diagrams, the rectangle symbol is used to represent entity sets while the ellipse symbol is used to represent attributes. The line is generally used for linking attributes to entity sets and entity sets to relationship sets.

The entity relation diagram is used to represent the entire information system for easy management of resources. The diagram can make the people concerned easily identify concepts or entities which exist in the whole information system as well as the entire business structure and the complex interrelationships between them.

An entity relation diagram is also often used in visualizing a relational database. Each of the entities represent the database table while the relationships lines represent the keys in one of the tables pointing to a specific record in the related tables or tables depending on the kinds of relationship (one to one, one to many, many to one, many to many).

An entity relation diagram could also be an abstract representation of something which does necessarily mean capturing every table needed within the database but is instead serving to diagram major concepts and relationships.

It may represent very industry specific theoretical overview of the major entities and relationships needed for management of the industry resources whatever they may be. It may assist in the designing process of the database for an e-resource management system but may not necessarily identify every table which would be used.

The "Crow’s Foot" notation is an alternative entity relations diagram. In this diagram scheme, the relationships are represented with connecting lines between entities and the symbols at the ends of the lines are to represent the cardinality of the relationship.

In representing cardinality, the "Crow’s Foot" notation uses three symbols: the ring represent zero; the dash represents one; and the crow’s foot represents more or many.

In representing the four types of cardinality which an entity could have in a relationship, the "Crow’s Foot" uses the following symbols: ring and dash represents zero or one; dash and dash represents exactly one; ring and crow’s feet represents zero or more; dash and crow’s feet represents one of more.

This diagram scheme may not be as famous and widely used as the symbols above but it is fast gaining notice especially now that it is used with Oracle texts and in some visual diagram and flowcharting tools such as Visio and PowerDesigner.

Those who prefer using the "Crow’s Foot" notation say that this technique give better clarity in the identification of the many, or child, side of the relationship as compared to other techniques. This scheme also gives more concise notation for identifying mandatory relationship with the use of perpendicular bar, or an optional relation, or an open circle.

There are many tools for entity relation diagrams available in the market or the internet today. The proprietary tools include Oracle Designer, SILVERRUN ModelSphere, SmartDraw, CA ERwin Data Modeler, DB Visual ARCHITECT, Microsoft Visio, owerDesigner and ER/Studio. For those who want free tools, their choices include MySQL Workbench, Open System Architect, DBDesigner and Ferret.