Star Fox 2/Gameplay

Easy

 * You will have to destroy only two bases and two battleships.
 * Andross will send out only one boss to fight you.
 * Andrew of Star Wolf will not appear.
 * The cores of battleships will only have two targets to destroy instead of four.
 * Viruses will not appear.
 * Battleships will sometimes fire the Planet Cannon.

Normal

 * You will have to destroy three bases and four battleships.
 * Andrew of Star Wolf will appear.
 * The cores of battleships will have four targets to destroy.
 * Viruses will appear. These are annoying, fast little enemies that will seize control of the satellite if they can reach it.
 * Battleships will fire the Planet Cannon more often.

Expert

 * You will have to destroy six bases and four battleships.
 * Andross will send out two bosses to fight you.

With each difficulty level, the enemies you encounter will be harder to defeat, especially the bosses. Andross in particular will have one more battle phase for each difficulty level. Also, each level is different on each difficulty setting. For example, you only fight King Dodora on Hard (but not on Expert). There may be other differences as well.