Well, you answered your question - they are nouns and adjectives and legal and legitimate will be used to describe a noun. A legal question. A legitimate complaint. If you mean, what is the difference in meaning between legal/legality and legitimacy/legitimate, both mean "according to law" but legitimate also has a wider meaning in some circumstances - for example, if we say "you asked a legitimate question" we mean the question was justified or appropriate. My complaint might be legitimate in the sense of being reasonable and justified but it might not be "legal" because the law does not permit me to complain. The best way to grasp the difference is to look up instances of the words and try and see the context in which each is used.