I just came across this superb introduction produced by the Cambridge Social Ontology group:
The term ontology3 derives from Greek, with “onto” meaning “being”, and “logos” usually interpreted as “science”; so that ontology, as traditionally understood, is the science or study of being4.
The word being has at least two senses:
1) Something that is, or exists;
2) What it is to be or to exist;
It follows that if ontology is the study of being it includes at least the following:
1) The study of what is, or what exists, including the study of the nature of specific existents
2) The study of how existents exist. This twofold conception is adopted here