A probability tree is only used to find probabilities. Once you are done with it, you can't change it into something else. Once you map out a decision using a decision tree, you can assign probability values to various consequences of the decision you mapped, effectively turning it into a probability tree. A decision tree can become a probability tree, but a probability tree can't become a decision tree.
A probability tree has its values already written on it. A decision tree has its values assigned later, once the entire decision process has been mapped out.
A probability tree has a fixed formula: when you move across a probability tree, you multiply the values, and when you move down the probability tree you add the values. A decision tree, on the other hand, doesn't necessarily have a formula. Rather, it's a way of displaying information; you don't have to follow any specific mathematical function or even include numbers at all.
A probability tree can compare multiple outcomes. You can use a probability tree to compare the odds of one thing happening in spite of the other or the odds of both things happening. A decision tree, on the other hand, tends to make things a little more specific. Since you are calculating decisions in a decision tree, the implication is that by choosing one thing you are choosing not to do another. In this way, probability trees give more information.