The Arduino hardware is simply a microcontroller (an 8-bit Atmel job in its original form) with a bootloader mounted on a somewhat standardized form factor PCB with headers. There are many different versions of "Arduino" based on many different microcontrollers, from 8 all the way to 32 bits I believe.

It's not running an interpreter but compiled/assembled code from the Arduino IDE (which I believe is built upon the gcc compiler). You can download the bootloader and flash it into a blank microcontroller using a device programmer (which can actually be another Arduino) and it "becomes" an Arduino.

So no, the Arduino is not similar to a BASIC Stamp (which is running a propritary interpreter) and you can buy blank microcontrollers from "anywhere" and program the Arduino bootloader into them yourself.