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
If you look at the options as "variables" used for "calculating" validity, never, marginal, and full are all clear. However, if so, I'm not sure how ultimate fits into the picture during the calculations.
What is the difference between the last two in the list?