An Entity Relationship Diagram (or ERD) is a diagram which is used to find links (or relationship) between one entity and other entities (normally within a database). The diagram consists of three different types of information: entities, the attributes of the entity, and the relationship between the entities.
Entities are often a noun (people, places, or things). Relationships are typically verbs (naming the action which takes place between one entity and another).
An example of an ERD could be the diagramming which takes place when trying to tie one person to another in a crime (notoriously used in finding a serial killer in movies and on television). A victim is tied to other victims using the verbs to highlight their relationships. Eventually, the information gathered can allow the potential person of interest to fall into the diagram based upon the similarities between those killed.
A visual ERD has been provided in the link below.