Why is this piece called a "cheese slope"? "Slope" I get; but "cheese"?
A cheese slope resembles a chunk of cheese from a cheese wheel. They happen to also be the perfect size for minifigures to interact with:
‘Lego Cheese Farm’ by AIatariel, http://www.flickr.com/photos/alatariel1181/10582728886/
As was mentioned in another answer, the cheese slope looks like a slice of cheese at around minifigure scale. This part has also actually been used as cheese in official sets, including Medieval Market Village: