Within your online CiviContribute donation form, you may let your constituents make pledges for future donations. By allowing pledges, donors have the flexibility to choose the frequency with which they will donate, e.g. twice a year, twice a month for five months, etc. Some payment processors will not allow this kind of automation and the payments themselves may arrive in a non-electronic form like cheques. However, you can record future income against a current source such as an event or email campaign, and change the status from 'pledged' to 'complete/paid' manually when the payment arrives (should the payment method not support this). Pledges also provide an automated mechanism to remind donors of their commitment.
For further information on pledges, and creating pledges internally on behalf of constituents (i.e. not through an online form), see the chapter 'Pledges'.
Setting up pledges for online forms is a simple check-box affair. Once a contribution page has been created (see the chapter 'Setup'), while editing the page's settings go to the 'Amounts' tab and tick the option 'Pledges'. Several options will appear, as below.
Once enabled, the option to make a pledge will appear on the online form. To check and monitor the current pledges, several reports and a dashboard are available. To access the dashboard, go to: Contributions > Pledges > Dashboard.