According to a short and sweet article in Cooking Lightabout cooking caramel, the problem is that sugar behaves differently from other foods when it's cooked. instead of absorbing heat from the pan like most ingredients do as they cook, sugar actually generates its own heat as it breaks down and causes the temperature to rise fast. When you remove the pan from the heat as the caramel reaches the perfect light-amber hue, it can still burn because residual heat from the pan keeps the action going.
The key is watchful, hands-off cooking, as slow and even as possible. Adding 1/4 cup of water per cup of sugar dissolves the sugar uniformly and slows boiling, providing more control as you look for that honey-gold color. Use a light-colored stainless steel or enamel saucepan and a candy thermometer.
To make the caramel, cook the sugar and water, without stirring (or absolutely minimal stirring, if you must), over medium-low heat until golden and fragrant, about 335°. The hands-off approach works best because stirring can cause hot caramel to crystallize when it hits the cool sides of the pan, and that can set off a chain reaction that ruins the sauce.
For pan cleaning, as counter-intuitive as this may seem, soak the pan in cold, not, hot water. The caramel will dissolve in the cold water but keep cooking and hardening in hot water!
For more details about making perfect caramel plus a photo, see the article on CookingLight.com.