The E-R Model:
An entity-relationship represents an excerpt of the world as a collection of entities (basic objects) and relationships among these objects.
Each entity has associated with it a set of attributes describing it.
The set of all entities or relationships of the same type is called the entity
set or relationship set.
The E-R Diagram:
E-R diagrams are used to graphically express an E-R Model
Rectangles represent entity sets
Ellipses represent attributes.
Diamonds represent relationships among entity sets
Lines link attributes to entity sets and entity sets to relationships
On the right hand side of this page you can see a sample E-R diagram.
Click at "Show Solution" to see a description of the E-R diagram..
Source: http://www.cs.sfu.ca/CC/354/zaiane/material/notes/Chapter1/node7.html