What Causes Dark Pigmentation Of Skin On Exposure To Sun?

Hello. I went to Africa where it s sunny everyday and lived there for about 7 years. I used to go outside and play in the sun daily and this caused my face and forearms to get really dark compared to my chest and other parts of my body. What is this called? How can I get back to my normal skin tone? This is really bothering me as when I take my shirt off it looks really odd. Thanks.