All forms of skin hyperpigmentation essentially involve the same mechanism: Darkening of the skin due to the over-production of melanin (skin's pigment substance) by overactive pigment cells called melanocytes. However, various factors can provoke melanocytes to go into overdrive, and these different root causes are what distinguish the different types of brown spots. Melasma, freckles and age spots are just different names for the different characteristics of skin hyperpigmentation.
Melasma or chloasma is a brownish discoloration of the face (commonly across the cheeks) that occurs most often in women. It affects all races, but is more common in people with darker complexions. Melasma generally arises in females after puberty and is associated with hormonal changes, especially during pregnancy. It is also aggravated by sun exposure and may become permanent if not treated early.
Freckles are small peppery brown spots arising on the face and other sun-exposed areas. They are usually seen in children and in fair-skinned people. As a person ages, they generally fade away. "Sunburn freckles" are another variety often seen in some people, and these appear larger and darker than standard freckles, with a medium-brown color. The edges are irregular, but the pigment is even throughout the area.
Age or liver spots are small, flat pigmentated spots which look similar to freckles. These are most often seen on sun-exposed skin after age 40, usually on the face, shoulders, neck, ear or back of the hands. Like freckles, age spots are primarily caused by sun exposure, but poor nutrition and abnormal liver function are said to worsen the problem. However, unlike freckles, age spots generally don't fade without treatment.