Introduction to the Semantic Data Model
The Semantic Data Model (SDM), like other data models, is a way of structuring data to represent it in a logical way. SDM differs from other data models, however, in that it focuses on providing more meaning of the data itself, rather than solely or primarily on the relationships and attributes of the data.
SDM provides a high-level understanding of the data by abstracting it further away from the physical aspects of data storage. 
Representing the Real World with Entities in SDM
In SDM, an entity represents some aspect or item in the real world, such as an employee. An entity is akin to a record in a relational system or an object in an object-oriented system. These entities in SDM focus on types, which are more general, instead of sets of data. In SDM, an entity is a very basic notion of a real-world or conceptual object that is defined by a single attribute.
For instance, an SDM entity type might be person, which provides an elementary category that can be easily understood. In a relational model, however, you might end up with a number of different tables “including person, spouse, children, house, and job.” Each of these things represents part of what makes up the person, but with SDM, the person is the whole entity, rather than breaking it down into parts.
In this way, an entity in SDM is very similar to a domain. Therefore, inside this domain of person, there would be a list of names of people that are to be represented by the data. The objects in this domain would then point to specific instances of a person that are represented by each person entity. For example, the domain Person names contains Bob, Sue, Jim, Betty, and Clyde. Each of these names points to a specific ojbect instance of Person, so that Bob points to a record giving details about Bob, such as name, gender, or marital status, and so on for each of the entities listed under Person.  
1. Jack, Colin, and Colin Keillor. "Semantic Data Models." Unpublished essay. School of Computing, Napier University. 15 Oct. 2006 <http://www.soc.napier.ac.uk/module.php3?op=getresource&cloaking=no&resourceid=7462386>.
2. Semantic Data Model. 13 Aug. 2001. COMTECO Ltd. 15 Oct. 2006 <http://www.comteco.ru/EN/DATAMODE/datamode.htm>.