You sign up on Kiva's site, choose a business, then pay in your loan amount through PayPal.
This is a charitable loan -- you will not receive interest on it. (Kiva does charge the borrower interest in order to recoup their operating costs.) When the money is repaid, you can take it back, or lend it to another business. You'll receive email updates about the business you lent to. And $25, which is probably just a week's worth of milk chai for some of us, will make a big difference to an entrepreneur.
Kiva seem to be experiencing some activity! When I first went to their site, I found a business that I thought I would like to support. But I wanted to do more research on Kiva first and, anyhow, pay day isn't until Monday. And now, an hour later, that business has met its $300 loan amount already.
Do your own research before giving or lending money to any organization. I only just heard about Kiva this afternoon, so this is not an endorsement from me, just some info.