An entity-relationship model (or ER model) describes interrelated things of interest in a specific domain of knowledge. A basic ER model is composed of entity types (which classify the things of interest) and specifies relationships that can exist between entities (instances of those entity types). Entity Relationship Modeling is a database modeling technique that aims at establishing a diagram or visual representation of a system's data.
There are three basic elements in an ER Diagram: entity, attribute, relationship. There are more elements which are based on the main elements, which we'll not focus on right now.
Here is an example of an ER Model: