My 100x objective is called a wet objective. What does this mean? What is oil immersion and why is it important?

A wet objective means it operates with oil between the lens and the specimen. Oil concentrates light from the condenser and reduces light losses from reflection and refraction. Oil immersion is the technique of using a drop of oil to wet the top of the specimen or slide cover and the front of the objective lens. This effectively immerses or bathes the light path between the lens and object viewed, allowing finer details to be seen. Both numerical aperture and resolution of your microscope are increased. 

Oil immersion is only used with the highest power objectives expressly designed as oil immersion lenses. Special oils are required so as to not damage the lens. The oil should have the same index of refraction as the objective. Your lens should specify what kind of oil should be used with it, either Type A (low-viscosity oil) or Type B (high-viscosity oil).

Updated 12/18/13