You actually just made cheese. The curds are the cheese. When you add an acid like vinegar or lemon juice to milk, you change the shape of the casein. Casein is a protein. In its changed state, casein clumps together, or coagulates, to form solid curds.
Heat helps separate the curds from the liquid whey.
Curd contains most of the fat, casein protein and vitamin A of the original milk. But whey has important nutrients, too. Dried whey is sold as an additive for bread, ice cream, processed luncheon meats and even food for animals.