• one-to-one: one instance of an entity (A) is associated with one other instance of another entity (B). For example, in a database of employees, each employee name (A) is associated with only one social security number (B).
• one-to-many: one instance of an entity (A) is associated with zero, one or many instances of another entity (B), but for one instance of entity B there is only one instance of entity A. For example, for a company with all employees working in one building, the building name (A) is associated with many different employees (B), but those employees all share the same singular association with entity A.
• many-to-many: one instance of an entity (A) is associated with one, zero or many instances of another entity (B), and one instance of entity B is associated with one, zero or many instances of entity A. For example, for a company in which all of its employees work on multiple projects, each instance of an employee (A) is associated with many instances of a project (B), and at the same time, each instance of a project (B) has multiple employees (A) associated with it.

Entity-Relationship Models
A brief overview from Lonsdale Systems.

How To Draw Entity Relationship Diagrams
A tutorial using SmartDraw, a data-modeling application.

Introduction to Data Modeling
This document is an informal introduction to data modeling using the entity-relationship approach. It is intended for someone who is familiar with relational databases but who has no experience in data modeling. The basic techniques described are applicable to the development of microcomputer based relational database applications as well as those who use relational database servers such as MS SQL Server or Oracle.