I personally prefer leather, but there's another forum kicking around here somewhere where this debate is better hashed out...
Seriously, it's a sign of a strong head, really good head retention. It's the white foam that clings to the top of your glass as you down the pint. It's a thing of beauty and a matter of pride; don't you love it when you drink a beer that's got a nice head, one that's nice and tightly formed, nice and smooth and creamy, one that doesn't dissipate as soon as you take a drink?