Although both terror and horror imply extreme fear or the presence of danger or evil, they have a slight difference in usage. Terror is typically used in anticipation of a danger that's in the future. This intense dread or panic is a reaction to something that could or is going to happen.
Terror seeks to evoke by suggestion, by dreadful suspense; horror displays in the hopes of generating revulsion. Terror veils a potentially ghastly unknown and tempts the reader to advance cautiously and with anticipation, while marches the reader through a landscape of unmistakable taboo and atrocity.