G'day all.
Oh, one more thing.
Quoting Aaron McDaid
Somebody more knowledgeable can describe the etymology of the terms, [...]
You can think of a type as a set of values. For example, Bool is the set { False, True }. A "class", then, is a set of types. The distinction between "set" and "class" comes from the various set theories (Goedel-Bernays-von Neumann set theory being the most common) which try to avoid Russell's Paradox. For those who are don't know about Russell's Paradox, take a look at the Wikipedia entry before going on: http://en.wikipedia.org/wiki/Russell%27s_paradox The idea behind GBN set theory is to distinguish between "sets", which are always well-behaved, and "classes", which are not necessarily so well-behaved. Russell's Paradox is resolved by setting up your axioms such that the paradoxical "set of all sets with property X" is not, itself, a set, but a class. By analogy, we call a set of types, or a "set of sets", a "class". Cheers, Andrew Bromage