Mandriva Linux was first released in 1998 (under the name "Mandrake Linux") as a modified version of RedHat linux, with KDE added. Since that time Mandriva has diverged to form it's own mainstream distribution with the only real relationship to RedHat/Fedora being the use of RPMs as the package format.
Mandriva Linux is designed for the desktop, but also works well as a server and in various other fields. Being aimed at the desktop, it includes a number of easy to use tools to configure it and keep it up to date. These are all under a GPL-like licence or the GPL itself.
Mandriva Linux was one of the first main-stream distributions to move to a community development model, similar in many ways to Debian's model. This move was later copied by RedHat when they moved to Fedora.
Mandriva currently supports x86, PPC, and AMD64 versions of Mandriva Linux. x86 and AMD64 have current Official releases. While the Current Official version for PPC is 9.1, there is a PPC branch of Mandriva's development version (Cooker) which is fairly current. At various times Mandriva has produced a Sparc and IA64 version, though neither appear to be maintained at this time.
Some of the tools are:
urpmi is a most useful tool for Mandriva Linux, and setting it up should be one of the first things you do. This can
be done from the command-line (follow the instructions at 
http://easyurpmi.zarb.orgx), or via the RPMdrake system
(accessed via the menus).
Documentation: As for most distributions, the best way to learn Mandriva is from the distribution's own documentation: