Xbox 360 slim 250GB HDD help please?

alright guys, so i wanted to buy a 360 slim 250gb HDD but i was looking and i found external and internal drives what is the difference between external and internal drives?? i just want a Hard drive for my 360 slim to save my games on and music and i just want to put it in my 360 and just work from the start and i don't have to mess around with it when i get it or need to install it or transfer my games from my console to the hard drive please help!! thanks!!