I am entering the world of ontologies for my phd thesis. That is why I put some defintions here. According to the W3C:

An ontology defines the terms used to describe and represent an area of knowledge. Ontologies are used by people, databases, and applications that need to share domain information (a domain is just a specific subject area or area of knowledge, like medicine, tool manufacturing, real estate, automobile repair, financial management, etc.). Ontologies include computer-usable definitions of basic concepts in the domain and the relationships among them (note that here and throughout this document, definition is not used in the technical sense understood by logicians). They encode knowledge in a domain and also knowledge that spans domains. In this way, they make that knowledge reusable.

Then, as stated by Tom Gruber, an ontology is a description (like a formal specification of a program) of the concepts and relationships that can exist for an agent or a community of agents. And What is important is what an ontology is for.

One of the purpose of my phd is to establish a method to analyze mobile collaboration as well as mutual modeling (a process that occur during collaboration). In order to meet this end, I am considering developing an ontology that can describe the task and the agents activity.