An ontology is a set of precise descriptive statements about some part of the world (usually referred to as the domain of interest or the subject matter of the ontology). Precise descriptions satisfy several purposes: most notably, they prevent misunderstandings in human communication and they ensure that software behaves in a uniform, predictable way and works well with other software.What is OWL 2 OWL 2 Primer
Classes are used to group individuals that have something in common in order to refer to them. Hence, classes essentially represent sets of individuals. In modeling, classes are often used to denote the set of objects comprised by a concept of human thinking, like the concept person or the concept woman.Classes and Instances OWL 2 Primer
In OWLGrEd notation, classes are represented by yellow rectangles with the name of the class at the top.