What's the difference between trusting a key and signing it?

Question

What's the difference of meaning between trusting (trust command) a key and signing (sign command) it?

Answer 1

Trusting a key means that you will accept signatures from it. Signing a key means expressing that you have checked that the user really belongs to that key.

So, for example, if I sign your key, I am saying (to myself as well as to others) that this really is your key. That doesn't necessarily mean that I trust a signature from you on someone else's key. That only happens if I mark your key as trusted.