Correct spelling for the English word "art" is [ˈɑːt], [ˈɑːt], [ˈɑː_t]] (IPA phonetic alphabet).
The second person singular, indicative mode, present tense, of the substantive verb Be; but formed after the analogy of the plural are, with the ending -t, as in thou shalt, wilt, orig. an ending of the second person sing. pret. Cf. Be. Now used only in solemn or poetical style.