They're actually very similar, but a protein shake is essentially a protein supplement, whereas a meal replacement is specifically formulated to replace a meal. In other words, a meal replacement is basically a protein shake that's fortified with additional vitamins & minerals, dietary fiber (for improving regularity), essential fatty acids (also called "EFAs" – Omega 3s, 6s & 9s - for healthy skin, hair & cognitive development), and complex carbohydrates (for sustained energy).