As adjectives, both mean "inside", although they collocate with different types of nouns. Alan's suggestion of looking up some sentence examples in dictionaries is a good idea to help get familiar with which nouns they typically modify, respectively.
"Interior" can also be a noun, while "internal" can't (it's always an adjective). Also, "internal" has a corresponding adverb, "internally," while "interior" has no corresponding adverb.