What is the difference between the Oval Window and the Round Window?

The oval window (or vestibular window) is a membrane-covered opening which leads from the middle ear to the vestibule of the inner ear.

Vibrations that come into contact with the tympanic membrane travel through the three ossicles and into the inner ear. The oval window is the intersection of the middle ear with the inner ear, and is directly contacted by the stapes; by the time vibrations reach the oval window, they have been amplified over twenty times from what they were when they contacted the tympanic membrane, a testament to the amplifying power of the middle ear.
The round window is one of the two openings from the middle ear into the inner ear. It is sealed by the round window membrane, which vibrates with opposite phase to vibrations entering the inner ear through the oval window. It allows fluid in the cochlea to move, which in turn ensures that hair cells of the basilar membrane will be stimulated and that audition will occur.