DatatypeProperty and AnnotationProperty are not the same, especially in OWL (2) DL. In OWL (2) DL, terms defined in an ontology are separated into classes, datatypes, object properties, datatype properties, annotation properties, instances and data values. An ObjectProperty MUST relate two instances. A DatatypeProperty MUST relate an instance to a data value. Therefore, in OWL (2) DL, you cannot use a DatatypeProperty or an ObjectProperty on a class or a property. This is why there are AnnotationProperties, which can relate any entity (except the reserved ones) to any kind of values (either data value, instance or even class or property). This works in OWL (2) DL because the semantics explicitly says that annotation properties should be simply disregarded in the reasoning process. It is thus a convenient way to add information to classes, properties or ontologies. You should know that in OWL (2) DL, AnnotationProperty, DatatypeProperty and ObjectProperty are pairwise disjoint.

In OWL Full, however, these considerations are not very relevant because you can arbitrarily use a DatatypeProperty or an ObjectProperty on a class or property.

DatatypeProperty and AnnotationProperty are not the same, especially in OWL (2) DL. In OWL (2) DL, terms defined in an ontology are separated into classes, datatypes, object properties, datatype properties, annotation properties, instances and data values. An ObjectProperty MUST relate two instances. A DatatypeProperty MUST relate an instance to a data value. Therefore, in OWL (2) DL, you cannot use a DatatypeProperty or an ObjectProperty on a class or a property. This is why there are AnnotationProperties, which can relate any entity (except the reserved ones) to any kind of values (either data value, instance or even class or property). This works in OWL (2) DL because the semantics explicitly says that annotation properties should be simply disregarded in the reasoning process. It is thus a convenient way to add information to classes, properties or ontologies. You should know that in OWL (2) DL, AnnotationProperty, DatatypeProperty and ObjectProperty are pairwise disjoint.

In OWL Full, however, these considerations are not very relevant because you can arbitrarily use a DatatypeProperty or an ObjectProperty on a class or property.

There are significant differences in OWL DL concerning modeling and reasoning.

Modeling: For annotation properties, you cannot place any axioms (OWL 1 DL) or only few axioms (OWL 2 DL: subsumption, domain and range) on them, and you cannot use them in class or property expressions, such as property restrictions. You can use data properties in much more scenarios. On the other hand, you can use an annotation property with all of literals, URIs, and blank nodes as its values, even in the same ontology. In OWL DL, data properties can only be used with literals as its values.

Reasoning: Annotation properties have no semantics and are therefore completely ignored by a reasoner (at least in OWL /2/ DL). On the other hand, data properties have semantics and are used by the reasoner to infer new information.

These restrictions do not hold for OWL Full. There, annotation properties /have/ semantics and are therefore /not/ ignored by the reasoner (they are special object properties). And data properties can also have URIs and blank nodes as their values (they are special object properties as well). The difference there is that you can infer from the application of a data property that the value must be a data value (an instance of rdfs:Literal), even if it is a URI or blank node. You cannot infer this from the use of an annotation property.

There are significant differences in OWL DL concerning modeling and reasoning.

Modeling: For annotation properties, you cannot place any axioms (OWL 1 DL) or only few axioms (OWL 2 DL: subsumption, domain and range) on them, and you cannot use them in class or property expressions, such as property restrictions. You can use data properties in much more scenarios. On the other hand, you can use an annotation property with all of literals, URIs, and blank nodes as its values, even in the same ontology. In OWL DL, data properties can only be used with literals as its values.

Reasoning: Annotation properties have no semantics and are therefore completely ignored by a reasoner (at least in OWL /2/ DL). On the other hand, data properties have semantics and are used by the reasoner to infer new information.

These restrictions do not hold for OWL Full. There, annotation properties /have/ semantics and are therefore /not/ ignored by the reasoner (they are special object properties). And data properties can also have URIs and blank nodes as their values (they are special object properties as well). The difference there is that you can infer from the application of a data property that the value must be a data value (an instance of rdfs:Literal), even if it is a URI or blank node. You cannot infer this from the use of an annotation property.