Properties are creates for classes and object. Properties basically expand the knowledge base and act as user derive relationship between classes and object. Class properties are Boolean expression between classes or regular expression; similarly object properties are regular expression of in built data type and object value or constant value. An object property between two classes can be formed as below
Figure 3. Ontology
Item hascost cost (i.e. milk hascost 40)
Where item and cost are two different classes and hascost is user defined object property, hascost maps two different classes. Owl script for this expression is as http://www.semanticweb.org/laptop/food#hascost -->
Inference is a kind of technique that is used for expansion and acquisition of knowledge. Inference is used to add new information using rules, previous data and metadata. OWL-DL is a type of inference method and reasoning technique that is based on Hemit, Fact++ and Pellet like reasoner. These inferences method provides a way to classify, realize and verify semantic data. These rules includes reflective, symmetric, transitive etc. A very simple semantic rule of transitive type is
(a isbrotherof c) and (a isbrotherof c) - > (b issibblingof c)
In above expression left hand side data is actual data in ontology whereas right hand side is derived data using inference method. This rule provides expansion of knowledge and act as DL query to get knowledge from ontology. DL inference will also works on graphical data and gives output in graph forms.