Slax can best be described as Linux-based operating system that runs off a CD or USB flash drive.
According to developer Tomáš Matějíček, Slax is a fully-functional OS that takes up less than 190MB of space and runs well on computers with at least 256MB of RAM.
Slax 7 RC 1 is based on the Linux 3.6.6 kernel paired with the KDE 4.9.2 desktop environment. Basic apps packaged with the OS include Firefox, a number of games, a document viewer, media players and some utilities.
However, as Liliputing's Brad Linder notes, what makes Slax particularly appealing to devs and modders is its modular approach to software.
Meaning, rather than downloading and installing apps one-by-one from a software store, you can simply add a Slax module to a flash drive and load it every time the OS boots or on an as needed basis.
Matějíček says he plans on selling 16GB USB flash drives loaded with Slax 7 for $25 later this year. In the meantime, you can give the release candidate a try by downloading the OS here in either 32-bit or 64-bit iterations and in ISO or TAR versions.
The ISO version can be easily burned to a CD and opened via a virtual machine like VirtualBox. Or, the TAR iteration of Slax 7 can be downloaded and unzipped, creating a bootable USB flash drive.