Explain the following tools for documenting and modelling user requirements Entity-relationship diagrams (ERDs)

Entity-relationship diagrams (ERDs)
An ERD is a graphical model that shows relationships among system entities
 Each entity is a rectangle, labelled with a noun
 Each relationship is a diamond, labelled with a verb
Types of relationships
 One-to-one (1:1)
 One-to-many (1:M)
 Many-to-many (M:N)
A full ERD shows all system relationships