My understanding of the difference:
Perfect pitch means someone can successfully produce a note without having heard it very recently. In the guitar tuning example above, this would be like producing the 'tunng' base note oneself, rather than using a tuning fork. Relative pitch means someone who can produce a note relative to the note produced by the tuning fork (or by someone who happens to have perfect pitch).
A good ear for music would refer to how well someone can recognise, or having heard a note, reproduce it perhaps on the same, or even a different, instrument.
Can't reliably do any of these myself and always felt that this was a disadvantage. DH can do all three and , as PPs have pointed out above, suffers horribly when things are out of tune. He is astonished that I hardly even notice and am certainly not bothered by out of tune things. I'm not completely tone deaf, just not very sensitive at all.
Understanding music and enjoying it adds so much to people's lives, am always glad to hear about people who do love, or can enjoy, it. Especially when DCs are being encouraged to enjoy music.