This is a 4GB microSDHC card (SDC44GB) that comes with a SD adapter and is pre-programmed to contain the latest Raspberry Pi Linux distribution "Raspbian Wheezy" which was released on 2013-02-09.
Raspbian is a free operating system based on Debian optimized for the Raspberry Pi hardware. An operating system is the set of basic programs and utilities that make your Raspberry Pi run. However, Raspbian provides more than a pure OS: it comes with over 35,000 packages, pre-compiled software bundled in a nice format for easy installation on your Raspberry Pi. You can read more about Raspbian here. These microSDHC cards use the new speed “class" rating of Class 4 that guarantee a minimum Data transfer rate of 4MB/sec. We may ship a Sandisk or Kingston card, both are high quality and come with the same speed class rating.
Adafruit tests and verifies each card before shipment. This image can be downloaded directly from the Raspberry Pi Website Downloads Page.