"Toss" is more gentle. You can only toss something to Bill if Bill is aware and ready to receive. It is not possible to toss something "at" Bill. However, you can "throw" something at Bill whether he is ready or not. You can even throw something that might hurt him. "Throw" is just a physical act. "Toss" is a bit like "hand". You can toss something to Bill just as you can hand something to Bill. The only difference is that when you "toss", it flies through the air. When you "hand", you place it in his hand.