Axel Inquirey

Sorry if I'm "green" on this subject, but I have an inquirey. I have lined up to purchase set of aftermarket axels for my car from an old buddy. I'm switching from a 4" bolt circle to a 4 1/2" circle. The axels in question haven't been installed into a housing, they are brand new. The seller tells me it has the "green" bearings. I've seen ads in magazines mention these green bearings. Sorry but what are they? And how do they differ to anything else.