When you sign up, you provide an email address. It's used for a couple purposes only. One is to make sure we have a place to send your forgotten password requests. The other is to facilitate responding to your questions or concerns through the Contact form. Obviously, you'll also need a valid email address to take advantage of email alerts (you control these settings; sent once per day to notify you about sales rank changes based on criteria you specify). Your address won't be sold or abused in any way.
All subscription plans are processed through PayPal. It's safe and easy. Rankforest has no information concerning your payment method; everything is handled on PayPal.com's secure servers.