From AT&T Support Wiki
An ontology is an explanation of terminology (concepts) and the relationships between them. To borrow from Robert Heinlein, "grokking the essence" of any deep subject is a matter of fully appreciating the ontology.
Of course, to paraphrase Clarke, Any sufficiently complex ontology is indistinguishable from gobbledygook. That is, there are so many terms, and so many relationships, that it becomes impossible to discern them all.
The solution, to borrow a concept from mathemetics, is a "partial ordering" -- a system in which you divide things up into subsets. And then (things get hazy here), either the subsets are ordered, or the items in each subset are ordered. In our case, we're concerned about ordering things within each subset--in other words, with making sense of terminology within subdomains of the whole system.
Of course, terminology will bridge the gap between subsets, at times. That is the nature of complex systems. But to the degree we can achieve a proper "separation of concerns", as Dysktra would say, we can help someone to more rapidly achieve a "mental ordering" that matches the actual underlying system.
Here are the ontology pages: