Jetsons, both TK1 and TX1, arrive with a full Ubuntu 14.04 LTS distribution. The distribution comes with a subdirectory “NVIDIA-INSTALLER”. By running the installer (“cd /home/ubuntu/NVIDIA-INSTALLER”, “sudo ./installer.sh”, where default password for ubuntu is literally “ubuntu”) the nVidia-specific hardware acceleration and access files are also installed. Without those files you have a full Ubuntu distribution, but much of the hardware is ignored and things slow down without taking advantage of Jetson-specific capabilities.
At the moment, the most current version of L4T for Jetson TK1 is R21.4, and is very much worthwhile flashing this (you would need a Linux host, Ubuntu 14.04 LTS has the most support). FYI, most instructions refer to your host also being Ubuntu, and for your desktop x86_64 machine, you would need to click the install if you’ve just created an Ubuntu DVD for installing on the x86_64 desktop host.
In terms of software you can download for things like flashing, “JetPack” is a bundle of many things which runs on your Ubuntu host. This can simplify life. If you just want to flash the Jetson with a new L4T version (e.g., R21.4), then you can just download the R21.4 by itself.
I would recommend against installing to the SSD until you know your way around flashing and setup. The internal solid state disk (eMMC) is 16GB, with roughly 15GB being available (some partitions exist which are used for other things and consume a small amount of space). The included eMMC disk is referred to as mmcblk0, while an external disk is referred to as mmcblk1 (or some software calls it /dev/sda). If you need to know the device to flash to for root partition, just choose “mmcblk0p1” for now (the “p1” is a partition within the mmcblk0 device).