An Entity Relational Diagram (ERD) is a visual Representation of Entities and how they are related to eachother.  An ERD has 3 parts to it; an entity which is a person, object, place or eventfor which data is gathered, a relationship is the association between the entities, and the cardinality which defines the relation between the entities in terms of numbers.

As I am very busy at the moment, I cannot spend as much time as I would like on here. However I should be pretty free after this week. Meanwhile watch these videos dealing with ERD and relationships.