You can make them mean whatever you want, just be consistant. For me, 'settle' means I want the dog find a place and lay down but he's allowed to move/shift/grab a toy and chew on it if he wants. I use 'down' to mean I want the dog to down in the location of my choosing and wait for further instructions (usually 'stay').
Emma's trainer (agility/ckc obediance) uses Down as is elbows on the ground in the position they can get up from quick.
She uses settle in a long down stay. The dog is still lying down but the weight is shifted onto one hip. It's morecomforetable for the dog wich means less liekly to get up.
In my obedience class, we use the same differentiation JennSLK posted above. "Down" is just a quick down, "settle" means "lay down and prepare to stay there for a while", such as the long down stay in obedience competitions.