Bootloading Atmega328p-pu with USBTiny AVR Pocket Programmer

Hi,
can some one explain maybe in short, what is actually going on, bootloading arduinos?
2 Usb connections! I finally got it working, but why so complicated?
When I bought the avr pocket programmer I thought thats all I would need, connect it to the ISP connector of the chip e.g.ATmega328P, add a resonator and programm it.
Instead of using an IDE I thought I`ll be using the Arduino Software.

I got it working, but don`t have a clue how.

Thanks for any explainations,

John :slight_smile: