To be startled means to be physically surprised and shocked, like when someone suddenly taps you on your shoulder while you're listening to your iPod with your eyes closed. Astounded is also a surprise, but not physical. More like when you can't believe someone did or said something, when you're left speechless. But it can be positive as well, you can be amazed with something well done. Appalled is similar to horrified or disgusted. Usually you feel that way when you see something that you don't often see like living conditions of the poor or someone's nasty behavior.