These two words can be difficult to remember because they sound the same and have related meanings.
Hurtle is a verb—or rather two verbs. Without an object, it means “move very quickly.” With an object, it means “throw something with force (so that it moves through the air very quickly)”:
The rocket hurtled towards the stars.
The soldier hurtled the hand-grenade towards the enemy forces.
Hurdle, on the other hand, can be a noun or a verb, and refers to an object that one must jump over, such as in a horse-race, or, metaphorically, any kind of obstacle (even an abstract one):
You must jump over many hurdles to get a Ph.D.
The verb to hurdle means “to jump over a hurdle” and it must always have an object:
The horses hurdled many barriers during the race.