Ontologies come under the field of Semantic Web (Artificial Intelligence). These are the representation of a particular domain. All the relationships , entities and individuals involved in the concept can be easily represented via an ontology. Ontologies are helpful as they help in the knowledge representation which can be shared and re-used across a particular domain.
Ontology of a particular domain constitutes the classes, properties and the individuals associated with it. The properties can be object property and data properties. Object property link the two classes with each other, so therefore they signify relationship between two classes. Data Properties link the classes with the data type they hold. This all can be explained through an example-
Consider we have to form the ontology of the domain- 'School'. The classes that can be included for this ontology are- People, Classroom, Departments, Location etc. These classes can further be divided into sub-classes. People class can have sub-classes as- Student, Faculty, Staff. Similarly, the department class can be sub-divided into sub-classes based on different departments.The object properties for the school domain can be - teaches : connecting two sub-classes Student and Teacher i.e Teacher teaches Student. Another object property can be- studiesIn : connecting Student and Classroom i.e Student studiesIn Classroom. Several other object properties can be constructed. The data properties for this domain can be - hasRollNumber : connecting Student with integer data type i.e Student hasRollNumber int. Similarly, hasName can be used to associate the name of the person with String data type.
Apart from representing classes and properties, individuals can also be represented in an ontology. Example- Ram (teacher) teaches Ajay (Student).
The ontologies can be driven by using a software called as Protege. Protege helps in constructing an ontology. The naming convention followed by the ontology is generally the lower camel case i.e the classes, individuals are written with the first letter as the capital letter while as the properties are written with the first word in lower case and the second words first alphabet in upper case. Example- hasRollNumber.
While constructing the properties, cardinality can also be specified that is whether the relation is 1:1 or n:1 or n:n.
When an ontology is made, it should be able to answer a particular set of competency questions. The competency questions are a set of questions that can be used to answer particular queries related to the ontology. These are used to check the efficiency of the ontology.
Example- For the School domain, the competency questions can be- Where is the school? Where does the student study/play?
So, ontologies are helpful in sharing and re-using of knowledge. These represent the concepts of a domain via the relationships between the classes.