Within your online CiviContribute donation form, you may allow your constituents to make pledges for future donations. By allowing pledges, donors have the flexibility to choose the frequency in how they will donate. For example, twice a year, twice a month for five months, etc. Some payment processors will not allow this kind of automation and the payments may come in as cheques, electronic payments or whatever else you allow. However, you can record future income against a current source such as an event or email campaign, etc. It also provides an automated mechanism to remind pledgers of their commitment.
Setting up Pledges is a simple part of configuring a contribution page. On the Amounts tab (second screen in the process), select the pledges checkbox and further options will appear like this:
Select the frequency and reminder options and allow other amounts if applicable, and the pledge option will appear on the contribution page as below.
Once you've started collecting pledges, you'll want to check on these through the Pledge home page. This is reached through the Contributions menu: Contributions -> Pledges -> Dashboard. and displays a summary of all pledges along with a list of individual pledges.
Then when you collect the payment for these pledges, you will need to visit the contribution page for the contact and click on the link at the top of the page entitled Click here to enter a pledge payment.