Like LKY's famous "Unity among diversity", these 2 distributions differ mainly on the packages of software that are chosen by the 2 companies. But besides that, both use the same stable Linux Kernel 2.4. Same set of common software.

But Mandrake 9.1 includes the following key software

based on Linux kernel 2.4.21 and includes XFree86 4.3.0, KDE 3.1, GNOME 2.2, OpenOffice.org 1.0.2, Mozilla 1.3, Apache 2, and Samba 2.2.7.

and Red Hat 9.0 comes with these

First introduction of new threading technology: NPTL (Native POSIX Thread Library)

OpenOffice.org office suite for creation of documents, spreadsheets and presentations.

Mozilla Web browser , email client, address book, HTML composer and authoring tool

Ximian EvolutionTM email client, calendar and contact manager

CUPS for drag-and-drop printing capabilities

Upgraded core components: Linux kernel 2.4.20, GCC 3.2.1, GNU libc 2.3 (with NPTL), web server powered by Apache 2.0


A piece of advice, if you're new to Linux, avoid Red Hat and use Mandrake.