A pledge is the promise to give a donation at a pre-arranged time in the future, either in full, or at regular intervals until a total goal is reached. For example, Sally pledges to give $3000 over five years, at regular monthly intervals of $50 each.
In CiviCRM, donors can make a pledge themselves through an online contribution form (see the 'Contributions' section), or internal users may do so on their behalf. Payments are handled separately, either manually (e.g. cheque) or automatically through a payment processor, and the pledge itself is the 'umbrella' that links all of the individual contributions together.
If a donor has missed a payment, the status of the pledge can be updated accordingly, and email reminders may be set for delivery automatically at specified intervals before the payments are due, to remind donors of their commitment.