USB vs Serial arduino?

I dont get the difference does it mean the code will have to be different?
also I have a USB parallax rfid reader and a USB arduino duemilanove If I get a USB host sheild are these compatible?

The serial Arduino is the "Old" Arduino from the days when PC's still had serial ports.

USB Arduino is the "new" Arduino, implementing the serial communication over a USB connection. It's still serial though, and programming wise there is no difference. The USB / serial is implemented by the tiny little FTDI chip on the (USB) Arduino board. On the PC there is a driver that implements a virtual serial port, which seen from a programming perspective is just like any "real" serial port.

Unless you specifically want to use your Arduino with a PC with a "real" serial port, you should get an Arduino board with a USB "serial port"

Sorry i don't know about the USB RFID reader.