What does each cellular connectivity indicator mean?

I'm used to "3G" and "E" on my AT&T iPhone 3GS, and I understand I may see "4G" on my new iPad when I enter a 4G area. However, I recently saw "o" on the new Verizon iPad, and had no idea what it meant. ### What network connectivity indicators can I expect to see on my iOS devices, and what do they generally mean? ### Are the connectivity indicators the same for all iOS devices (with expected carrier differences) or should I assume "3G" on the 1st gen iPad means something different than "3G" on the new iPad?