A donation is a sum of money given by a supporter to a project.
A pledge is a sum of money given by a supporter against a reward on offer by the project.
In both instances if the project is on 'all or nothing' funding and doesn't reach their target, the donations or pledges will be returned to the supporter and rewards will not be received.
If the project is on 'flexible funding' then the project creator will receive the funds that have been given. If a pledge was made then the supporter will receive the reward after the project closes.
Projects have an estimated delivery date under each reward on the project page. This date is determined by project creators as their best guess for when supporters will receive their rewards.