Well, there is a lot about the Raspberry Pi going on. It’s a funny little computer running on several linux versions, but most known is Raspian. With the latest model B, the Raspberry Pi 3 model B all out that is, you have a quad core ARM V7 computer on 900Mhz with 1Gb of ram and it doesn’t boot* without an micro SD card loaded with an .img file. And there we go. What is an .img file? .img stands in Linux lingo for an disk image. An image of a harddisk or CD, DVD. Even from a Flashdisk, an usb-stick that is, or micro- SD card. Now we have to write this image from file to SD card using a program or even more basic using dd in a terminal on another unix, linux or Mac Os X system. As my other computers are Mac’s I am going to use those.

So step 1: Find the Raspian system you need.

That is quite easy. Go to www.raspberrypi.org -> Downloads -> Raspbian and download:
Raspbian Jessie with PIXEL
Image with PIXEL desktop based on Debian Jessie
Version: September 2016
Release date: 2016-09-23
Kernel version: 4.4
Release notes: Link

or Lite depending if you want, a gui (Graphical User Interface) or bash prompt. (The file I downloaded was called 2016-09-23-raspbian-jessie.zip)

Further more this is the official site from the Raspberry Pi Foundation with loads of info and forums with al kind of questions and maybe more important answers!

step 2: Decompress or ‘unzip’ the downloaded zip file.

So how to decompress an ‘zipped’ file? Well under Mac OS X just double click and the system will take care of it. Leaving you with a file with an extension of .img