One person can find a joke a little funny, while someone else might find it downright hilarious. When something is super-funny, it's hilarious.
Hilarious is an adjective that describes a high degree of humor. Things that are funny can cause you to smile or giggle, but when something is hilarious, it causes you to laugh out loud. Of course, what's hilarious to me might not be to you. When I scribbled obscenities on your face while you were asleep? I found that totally hilarious. You? Not so much. Hilarious is also used sarcastically, when something isn't funny at all. Like when you woke up: you glared at me and said "Hilarious."