This 4GB SDHC card comes preinstalled with Raspbian Wheezy, a Linux Debian based operating system optimized to run on the Raspberry Pi.
It's nice and convenient to have it pre-loaded instead of having to download the image. Ultimately I ended up going to a 32GB card, but this was a nice starting point.
Overall, my MCM experience was great! They delivered fast and as promised. Great product, great price!