Thai sour curry (kaeng som) is sort of like the chicken soup of Thailand.  It’s popular everywhere and hundreds of variations exist throughout the country.  At its core it’s made from a super simple chili paste dissolved in broth (or water) and then simmered with fish or shrimp and vegetables. There’s no coconut milk in this soup to balance the heat and that’s one of the reasons it’s spicy.  The sourness comes from either tamarind or lime.
The southern version is unique in a few ways.  First, it uses turmeric which gives the paste and the soup a yellowish orange color.  As a result, sometimes it’s called “orange curry.” In some places in southern Thailand they also call it “yellow curry.”  This is different from the more common yellow coconut curry dish but which is still called “yellow curry” on Thai menus (eating in Thailand can be very confusing). Second, it’s really spicy. And third, there’s little if any sugar.