Your wish is my command. From my dictionary:
Pledge - A formal or solemn promise or agreement, esp to do or refrain from doing something.
Donation - Something (such as money, food, clothes, etc.) that you give in order to help a person or organization.
By Donation you simply give money to help and want nothing in return. By pledge you promise to make an agreement - in this case I've promised to buy a game with early access key and I've already make my payment. But there is an agreement behind a pledge but not behind a donation. That's the difference.
Example:
- You gave a homeless man a pizza. He is happy and sing a song for you. You've made a donation and you're now appreciated for it.
- You've said, that you'll give a homeless man a pizza if he sing a song for you. He did and he gets his tomato in return. That was pledge.
In first example the guy doesn't have to sing but in second he does (unless he doesn't want a pizza).