I'll leave explanation of the differences to others here, but I thought this information may be helpful to some. From
http://www.amazon.com/gp/help/custom...200505520&#usb:
Using Kindle While Charging Via USB
You can continue charging your Kindle via USB and still use it by unmounting or ejecting it to exit USB drive mode.
- Windows Vista and 7: Click on the Start button, select "Computer" from the options, right-click on the Kindle drive icon and select "Eject" from the pop-up menu.
- Windows XP: Right-click on the "Safely remove hardware" icon in the lower left-hand corner of the task bar and follow the on-screen instructions to remove Kindle.
- Mac OS: Control-click on the Kindle device icon and choose "Eject."
Kindle will then exit USB drive mode and display the Home screen. You'll see a lightning bolt on the battery status icon that indicates your Kindle is still charging.