Basically, Caterpillar adds customization options to a jailbroken iPhone. At present the tools include a Smart Dialer, customized wallpaper with calendar, customized SpringBoard backgrounds (the home page) and more. Interestingly, they're planning to offer the package on a subscription model and it will be "constantly growing as [they] add new tools and functionality."
The Caterpillar package is available now by adding their repository to Installer.app on a jailbroken iPhone. The service will run $20 for six months, $35 for 12 months, or $45 a non-expiring license. Of course, you jailbreak your iPhone at your own risk; I have not tested this myself.