A pledge is a promise to make a donation based on how a team or athlete performs during a game or an event. For example, a football fan can pledge for every touchdown her favorite player scores during a season. The football fan can pledge any amount for every touchdown scored and then her pledge will turn into a donation after game results are finalized.
A donation is simply an outright contribution to a fundraising campaign. For example, a baseball fan can make a one-time flat donation to his favorite baseball team. One-time flat donations are processed immediately.