What is the difference between ballet and classical ballet?

Do you mean the difference between neo-classical ballet and classical ballet?

If so, the differences can be very small or very large and depends on many factors such as choreography, music, what year it was created, and style. Neo-classical ballet tends to be more of a mix of contemporary/modern and ballet. Though the movement of the choreography may seem more contemporary like, you will notice that foundation of classical ballet positions, turnout, and posture is still being used throughout the dance. The music tends to be more down-to-earth or more upbeat compared to classical ballet music (example: Tchaikovsky). The costumes follow along with it (example: more of a newer and modern looking costume compared to romantic tutus or flat tutus.

If you are comparing romantic ballet to classical ballet, the difference will seem very insignificant to the untrained eye and many of the factors are almost identical. The feeling and look of romantic ballet is very floaty and soft (example: La Sylphide) and classical ballet tends to have more sharper and defined movement (example Sleeping Beauty). In romantic ballet you will see more longer floaty looking tutus; like dresses. In classical flat tutus are more popular. You will also notice in romantic ballet, the plots are a lot more fiction based and are very fantasized (example: The Wilis in Giselle). While there is still tons of imaginary creatures in classical ballet it is more acting in real life problems. There is practically very minimum difference between the two technique-wise. The term "classical ballet" is mainly used to just define the difference between it and neo-classical ballet.