The OpenPGP Standard defines 5 possible owner trust levels
|Not Calculated||No OwnerTrust value yet assigned or calculated.||-|
|None||Never trust this keyholder to correctly verify others signatures.||n|
|Marginal||Have marginal trust in the keyholders capability to sign other keys||m|
|Full||Assume that the key holder really knows how to sign keys.||f|
|Ultimate||Only used for keys that you own yourself||u|
As a general rule, you will trust most key owners Marginally, while you few of your good, long term trusted friends will by Fully trusted. You should never assign Ultimate trust to a keyholder other than yourself.
gpg does not automatically prompt you to assign trust to keys in your keyring. To update your trust values use the --update-trust-db option from the command line, or the trust command within the --edit-key prompt.
When listing keys, the OwnerTrust value is shown as the letter before the / in the trust output.