Milk is a mixture because it contains many food groups in it such as: Carbohydrates, proteins, vitamins, fats, salts and water etc.
Milk is a specific type of homogeneous mixture called a colloid. Examples of other commonly encountered colloids include prepared gelatin and fog. The particles in a colloid are large enough to see in direct light, but smaller than those found in a suspension.
Most milk sold in supermarkets as of 2014 undergoes a process called homogenization. Homogenization breaks up milk fat into smaller particles, allowing it to stay in solution. Fresh milk that has not undergone homogenization eventually separates, causing the cream to float to the top of the container and skim milk to settle on the bottom of the container.