Older releases for a TX1 were Ubuntu 14.04LTS, current is Ubuntu 16.04LTS. Note that these are sometimes referred to as “Linux for Tegra” (L4T)…L4T is just the original stock Ubuntu with direct hardware access libraries added (e.g., to take advantage of GPU). L4T R24.2.1 is the most recent version, and is Ubuntu 16.04LTS plus NVIDIA-specific hardware accel files.
The host used to flash a Jetson can be any x86_64 Linux desktop PC if flash is from command line. JetPack is a front-end, and can also manage packages, but requires the host to specifically be x86_64 Ubuntu 14.04. JetPack’s current version is 3, but does not directly install on the Jetson, so you’d still refer to the L4T version flashed to name what is on the Jetson (saying it is version 3 just mentions the installer and isn’t really what is on the Jetson).
On the Jetson itself you can see what L4T version it thinks it is via “head -n 1 /etc/nv_tegra_release”.