An Entity Relationship Model, also known as an Entity Relationship Diagram, a graphical representation of entities and their relationships to each other, typically used in computing in regard to the organization of data within databases or information systems. An entity is a piece of data; an object or concept about which data is stored. A relationship is how the data is shared between entities. The three types of relationships between entities are one-to-one, one-to-many, and many-to-many.
One-to-one is when one instance of an entity A is associated with one other instance of another entity B. One-to-many is when 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. Many-to-many is when 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.
A Semantic Object Model is developed from objects that are already known. This model is normally used to document a system, program, or object that is already in use and working properly. Another term for Semantic Model from the field of knowledge representation is "ontology": the set of classes, relationships, and functions in a universe of discourse.