The answer explains that the Greek alphabet consists of three categories: vowels, consonants, and semivowels. The first category, vowels, includes nine letters: alpha, epsilon, eta, iota, omicron, upsilon, omega, and the diphthongs alpha iota and epsilon iota. The second category, consonants, includes 14 letters: beta, gamma, delta, zeta, theta, kappa, lambda, mu, nu, xi, pi, rho, sigma, and tau. The third category, semivowels, includes two letters: upsilon and omega.