Cell Phone Internet Question?

I just got the new Sidekick 4G (first cell phone) and ive also got the 4G plan. Depending on where I am, the internet signal will either be a 4G, a G, or an E next to the three bars. I know what the 4G means (and is the fastest), but I cant figure out what the E and G mean. Neither are as fast as the 4G, but I cant figure out what they are/mean/stand for. A little help please??