What is the difference between thriller and horror movies?

In a thriller the main focus is suspense and tension whereas a horror film is trying to elicit fear, disgust and horror from its audience.

Thrillers are based on potential real-life (or sometimes actual) events and monsters: Hostage situations, Home Invasion/Rape, Plane Crashes, Bank Robberies, Car Chases, Stalkers, and many more scenarios. They can be scary and keep you at the edge of your seat, but have a potential real life theme, where as Horror is an extremely broad genre that can range from extreme gore to simply the unexplained. There is a thread of the (most-likely) impossible running through a horror tale: Supernatural, Ghosts, Spirits, Boogie Men, Magic, Killer Bunnies, and the like. The aim is not just to keep you at the edge of your seat but also to frighten the viewer/reader and are typically works of fiction.