The difference between the Fiio E11 and the E7 or E17 is that the E11 is nothing but an amplifier - you feed it a signal from a source (like the headphone jack of an Android phone or through the Line Out connector on an iPod) then the E11 amplifies that signal and sends it out to your headphone. The Fiio E7 and E17 also are amplifiers and can act just like the E11, but, the biggest difference is that they can also be DACs (Digital to Analog Converters) - this essentially means that they can act as external sound cards for your computer or (at least in terms of the E17) as an external sound decoder for anything that will output a digital signal, like a PS3's optical output or similar. If you had a computer that had a horrible sound card, then the E17 might be ideal for you because you could bypass the internal sound card on the computer, use the E17 to decode the music and convert it into a signal that can be piped to your headphones.
I've got a full review and demonstration on the E17 here if you want to see what I'm talking about (I actually hook it up to my PS3):
Hope that helps!
ah yes, so DAC definitely makes the sound better for he PC / laptop, but from what I read is that DAC cannot be used while on the go with iPhone / iPod ?