Look and Feel
A Look and Feel changes the complete visual presentation of an application, the.. look and feel so to speak. Selecting one of the look and feels applies it directly.
System Default
This is this the Look and Feel delivered with Java designed for the platform you are running on.
System Look and Feels
All the Look and Feels provided by Java, tailored for specific platforms.
Skin Look and Feels
Skins provided by the Skin Look and Feel project, found here.