First, let's clarify our terms.
a. Native apps are created in the target device's language; in iOS that would be Objective-C, for instance. This is what you want to avoid.
b. Web apps, on the other end of the spectrum, can be downloaded directly from web sites and will work on virtually any device, since they are simply web sites geared to the small screen.
I use PhoneGap Cordova to make apps that work in iOS and Android. To answer your questions from that perspective:
Hybrid apps will work on both devices, assuming you are hooking into features both devices possess, like a camera.
You may find this chart helpful to decide which framework to start learning: http://wiki.phonegap.com/w/page/36767911/UI%20Development%20on%20PhoneGap
I compiled a list of categorized links on one page to help me keep track of everything. Feel free to check it out: