They often can be used interchangeably. For instance: It was a terrible/horrible/awful movie. You see here any one of the three could be used. There is a slight difference, however. If I wanted to emphasize fear being involved, I would tend to use terrible. If I wanted to emphasize disgust, I would use horrible. For lesser degrees of negativity, where something or someone is simply bad in some way, I would use awful.

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The two words themselves mean almost the same thing, but here's the difference.. the root of Terrible is "Terror" and the root of "Awful" is "awe". you can say either one. "This burned food is awful. This burned food is terrible." Same meaning.

They are roughly the same level of bad. They are all worse than very bad!