Fearful, awful, dreadful, frightful, terrible, terrific, horrible, horrific, shocking, appalling are comparable in that all and especially their adverbs are used informally as intensives meaning little more than extreme (or extremely), but each term has a definite and distinct value when applied to a thing that stimulates an emotion in which fear or horror is in some degree an element.
Something is fearful which makes one afraid or alarmed. In literary or formal use the word usually implies a deep and painful emotion and a loss of courage in the face of possible or imminent danger.
In less formal English fearful may not imply apprehension of danger, but it may at least imply that the thing so qualified is a cause of disquiet.
Something is awful which impresses one so profoundly that one acts or feels as if under a spell or in the grip of its influence; the word often implies an emotion such as reverential fear or an overpowering awareness of might, majesty, or sublimity.
With somewhat weakened force awful may be applied to qualities or conditions which are unduly weighted with significance or which strike one forcibly as far above or beyond the normal.
Something is dreadful from which one shrinks in shuddering fear or in loathing. In weakened use dreadful is applicable to something from which one shrinks as disagreeable or as unpleasant to contemplate or endure.
Something is frightful which, for the moment at least, paralyzes one with fear or throws one into great alarm or consternation.
Frightful is also often employed without direct implication of fright, but in such use it imputes to the thing so qualified a capacity for startling the observer (as by its enormity, outrageousness, or its shocking quality).
Something is terrible which causes or is capable of causing extreme and agitating fear or which both induces fright or alarm and prolongs and intensifies it.
When the word carries no implication of terrifying or of capacity for terrifying, it usually suggests that the thing so described is almost unendurable in its excess (as of force or power) or too painful to be borne without alleviation or mitigation.
Something is terrific which is fitted or intended to inspire terror (as by its size, appearance, or potency). Terrific may be preferred to terrible when there is an implication of release of stored-up energy, physical, emotional, or intellectual, and of its stunning effect.
Something is horrible the sight of which induces not only fear or terror but also loathing and aversion; thus, a fearful precipice may not be horrible; in the practice of the ancient Greek dramatists, murder on the stage was avoided as horrible.
Horrible, like the other words, may be used in a weaker sense; in such cases it seldom suggests horror, but it does suggests hatefulness or hideousness.
Horrible emphasizes the effect produced on a person, horrific the possession of qualities or properties fitted or intended to produce that effect.
Something is shocking which startles or is capable of startling because it is contrary to one’s expectations, one’s standards of good taste, or one’s moral sense. Often in extended use shocking does not imply a capacity for startling so much as a blamable or reprehensible character.
Something is appalling which strikes one with dismay as well as with terror or horror. Sometimes appalling comes close to amazing but then retains the notion of dismaying and carries a stronger suggestion of dumbfounding than of surprising.