"a complete USB-based microcontoller development system, in a very small footprint, capable of implementing many types of projects."
It has it's own software environment, but can also run Arduino sketches using TeensyDuino:
Many type of sketches are able to run on the Teensy! The digitial I/O pins work with digitialWrite(pin,value), digitalRead(pin), and pinMode(pin,mode). The PWM pins can be used with analogWrite(pin,value). Eight (8) pins can be used as interrupts with attachInterrupt(). The Teensy++ has 8 analog inputs that can be used with analogRead().