Melanin is a pigment that is produced by specialized skin cells called keratinocytes. How much you have and how it's distributed determines your skin's tone. The more melanin you have, the darker your skin will be. Hormones and genetics determine how much of the pigment our bodies will produce, which explains the wide variety of skin tones that are present. Even two siblings who share the same parents can have different skin tones.
Melanin performs the important function of absorbing harmful ultraviolet rays from the sun. Our bodies produce extra melanin when we're exposed to sunlight, which is why people tan under the sun (or get burned). Light-skinned people have a greater tendency to get sunburned than dark-skinned people, as they don't have as much of the ray-absorbing melanin.
Because melanin is sometimes unevenly dispersed throughout the body, freckles can develop, most often in light-skinned people. Freckles are small, flat groupings of melanin-filled cells called melanocytes. Freckles develop through exposure to the sun and can be a range of colors.
Several attempts have been made to classify people's skin tones throughout the years. While other systems have fallen out of use, the Fitzpatrick Skin Type classification system is widely in use today. The system breaks people into six skin types according to hair and eye color, skin tone and propensity to burning under the sun [source: Commonwealth of Virginia].
Now that you understand the basics about why your skin looks the way it does, let's dive into some other skin mysteries -- like how it heals itself after being harmed.