WOFF Fonts, what Are they and why should I care?

Question

So mozilla has proposed a new webfont, i'm not really into that world, but i want to keep myself up2date. So whats the great thing since even Microsoft is backing it, why should I as a webdev care?

Whats the difference from the old ones? Do we need another font system? In what situations should i use WOFF? Why not just stick to the existing ones?

Also a new tag called WOFF might be useful.