Nose grease is an oily substance located on the surface of the human nose. A more scientifical name for it would be nasal sebum. Almost all human skin produces sebum, but it gathers mostly on the nose. When your nose is really oily, others can probably see how it shines light in every imaginable direction. You can also feel the oiliness if you touch your nose.
Nose grease has its uses, though. It can be used as a lubricant in an emergency where nothing else is available. It can be applied on skipping CD's in an attempt to fix it. And for all the beer lovers out there, this might interest you:
1. Pour a pint of beer so that it foams alot
2. Rub your index finger on your oily nose
3. Dip the finger in the beer
Now, watch what happens. The foam should go down pretty fast. Nice, isn't it? Don't recommend doing it with anyone else's beer, though...
Nose grease also has its bad sides. Especially if you need to wear glasses. Nose grease makes your nose slick and make your glasses slide down your nose. This can be fixed by washing your nose with some facial cleansing products, but it will only help for an hour or two.
Some disgusting people like to spread their nose grease on every possible place in public spaces. Windows, elevator doors, mirrors, you name it. It's just plain disgusting and not even funny. It's not nice to sit down at a greased up desk in the classroom.
Also, some people are actually able to squeeze the grease out their noses. Multiple white "worms" of grease will just come out of their pores. A cool trick to disgust some people at a bar or party.