Well, perfect pitch is the ability to name or sing a note out of nowhere, without a reference. Let's say you want to tune your guitar, and you can hear when it reaches every note's right on the spot. It's quite useful when you try to find out a progression, or when you are writting a tune, you can just write it down silently, directly to the staves...

Relative pitch is the ability to sing or hear a note (when you are given the interval) when you have a reference. Let's say someone plays you a C, and tells you to sings the minor third. (Eb in this case) That's something you can work on, since most pro singers have the ability to harmonize lines for example. I'm sure you have it a little too, can you name a minor chord from a major one? Well, if it's yes, develop it to the extreme, and you'll have relative pitch.