Except for package management systems and environments, I would like to know what the main differences are between them .
There are a few different ways to interpret "Redhat" here:
Red-hat Enterprise Linux (RHEL) - A specific, commercial distribution by Red Hat.
By far the biggest difference between Ubuntu and RHEL is the license terms - Red Hat Enterprise Linux is commercial.
You can, however, get basically the same distribution, but without the commercial contract, for free in the form of the CentOS distribution.
Distributions which are based on or inherit from Red Hat, which would also include CentOS and Fedora, and their derivatives.
It's actually very hard to compare Ubuntu against all those (RHEL, CentOS and Fedora) as if they are one thing, because they are all very different in themselves. They do all share the same package manager as I talk about below, but the differences between them are greater than the difference between their package manager and Ubuntu's.
RPM-based distributions in general - that is, distributions that use the Redhat Package Management system. This would expand your scope to include distributions like SUSE, Mandriva, PCLinuxOS, and their derivatives.
Ubuntu is based on Debian's package manager APT and DPKG. Red Hat, CentOS and Fedora are based on the Red Hat Linux package management system, RPM. Both package managers are now quite mature and have roughly equivalent features. You could debate individual design decisions though but it's hard to say that one overall is better than the other.
