An entity is an object in the real world that is distinguishable from other objects. Examples include the following: the Green Dragonzord toy, the toy department, the manager of the toy department, the home address of the manager of the toy department. It is often useful to identify a collection of similar entities. Such a collection is called an entity set.
An entity is described using a set of attributes. All entities in a given entity set have the same attributes; this is essentially what we mean by similar. Our choice of attributes reflects the level of detail at which we wish to represent information about entities.
For each attribute associated with an entity set, we must identify a domain of possible values. For example, the domain associated with the attribute name of Employees might be the set of 20-character strings. Further, for each entity set, we choose a key. A key is a minimal set of attributes whose values uniquely identify an entity in the set. There could be more than one candidate key; if so, we designate one of them as the primary key. For now we will assume that each entity set contains at least one set of attributes that uniquely identifies an entity in the entity set; that is, the set of attributes contains a key.