The aim of an information retrieval system is to find relevant
documents, thus relevance is a (if not 'the') central concept of
information retrieval. Notwithstanding its importance, and the huge
amounts of research on this topic in the past, relevance is not yet a
well understood concept, also because of an inconsistently used
terminology. In this paper, I try to clarify this issue, classifying
the various kinds of relevance. I show that: (i) there are many
kinds of relevance, not just one; (ii) these kinds can be classified
in a formally defined four dimensional space; and (iii) such
classification helps us to understand the nature of relevance and
relevance judgement.  Finally, the consequences of this classification
on the design and evaluation of information retrieval systems are
analysed.

KEYWORDS: Information retrieval, relevance, kinds of relevance,
relevance judgement, system design, system evaluation.