Linux can be adapted to a wide variety of environments, some even contradictory. For instance, you wouldn't want the same distro that runs a 64-core server on your PDA. You wouldn't want FlightLinux controlling your desktop. Some distros are designed to run well on very old or cheap hardware. Others are designed to showcase the latest accelerated desktops and gaming. Some are designed to be used only as LiveCDs, while others are designed to be installed using a traditional text-mode installer. Some are optimized for multimedia production or media centers (Ubuntu Studio, 64Studio, Mythbuntu).
There are also philosophical differences between the different distros. Some will offer or bundle proprietary (or closed-source) software (Xandros, Freespire), while others find the idea repulsive (Fedora, gNewSense, Debian.) Some believe that programs hsould be built from source and specifically tailored for each machine (Gentoo), while others believe that pre-built software should be offered as a matter of convenience (pretty much everybody else). Some are created by or sponsored by competing commercial backers (OpenSUSE, Ubuntu, Fedora, Xandros), while others are created by non-profit groups or individuals (Debian, Puppy Linux, Gentoo).
AutoCAD is not developed for Linux. Some versions will run in Wine, but most recent versions will not.
Not all version of Redhat Linux is free but you can download many desktop versions free.
They don't.
The question is too vague to answer clearly. There are too many versions of Linux and Unix and third party vendors.
Many distributions have older disk images in their archives. Try there.
Distributions
"Distros", or distributions.
Generally yes. Most Linux versions will work on a standard home PC, and many will work on one too old and slow to run Windows.
The versions are constantly changing, so look in the related link below for the latest information.
No.
Depends what you mean. There are hundreds of desktop distros, but linux is much more popular than you would imagine. Android, the Wii, almost all servers, most checkout monitors, and many public displays (airports, fast food menus, etc) all run linux. So, there are a few hundred distros, but I'd estimate there are over 10000 "versions" in use across the world.
Some versions of CorelDRAW can be run on Linux using Wine, while others can't.