trust a GnuPG key, you can choose one of these five options (and I'm assuming the same options exist in other OpenPGP tools):
1 = I don't know or won't say (undefined)
2 = I do NOT trust (never)
3 = I trust marginally
4 = I trust fully
5 = I trust ultimately
However, there is a "sixth" option that doesn't show up in the list: if you haven't yet set trust for a key, it shows up as unknown.
Is there a reason GnuPG makes a distinction between the two? Is undefined trust treated differently than unknown, or is the distinction just for personal reference?