This is an excellent thread. Some good advice has been tossed around, and some people's feelings have been hurt, namely those who are so entrenched in software piracy, they can't see the light of day.
It is unfortunate that Linux is compared directly to Windows. When most people say "Windows" they are generally referring to the latest version of Windows, or the version that they are running themselves. Most people are off XP, and Vista and 7 are similar enough they are easily lumped together, so it's really not THAT big of a deal.
However, when you say "Linux", nobody really ever means the same thing. One person's experience with a one distribution will typically cause them to feel the same way towards all Linux distros, when they are very very very different.
Let me see if I can shed a little light on the subject, with my personal experience.
RedHat Enterprise Linux (aka Red Hat, RHEL) is an Enterprise class Operating system, that you must pay for a support license from Redhat to be able to use. Since their Operating system, and much of their software is Open Source, the CentOS project is aimed at stripping all non-GPL content from RHEL, replacing it with their own branding, and building all the same RPMs. CentOS is really only run by a handful of people, who work purely on a volunteer basis. Redhat is mildly interested in the success of CentOS, and don't consider them a threat at all. Redhat is primarily a support company, not a software vendor.
Debian is also an Enterprise class Operating system, based more on BSD principles, rather than SysV like Redhat is. They follow the same types of trends as Redhat as far as release cycles go. They are also on Version 5, update 0. The version numbers between RedHat and Debian are completely unrelated.
Then there's the more desktop centric OS versions. Fedora is Redhat's desktop OS, but it is targeted at the System Admins of their RHEL products. Fedora is sort of a sneak peak at what RHEL will someday look like. They try to have the most bleeding edge version of whatever open source software may possibly have an impact on the RHEL platform. Fedora is not for the faint of heart. It is often so far ahead of the curve, support can be sparse. For example, not many people run a version of the kernel that is less than 2-3 months old. New features in that kernel aren't widely tested, but that's what Fedora is for!
Ubuntu is targeted as as Windows Replacement Desktop. I highly suggest the Ubuntu experience for any one interested in replacing their Desktop OS with a Linux distro. Ubuntu and Debian are both owned by Canonical. Canonical uses their Ubuntu Server distro very much like Redhat uses Fedora.
As far as the big dogs go, Ubuntu is where you start as a desktop user. If you're interested in making a career out of running Linux Systems, both Debian and RHEL are perfectly viable options, but Redhat seems to be more popular in the business place. As you learn more about Linux through use and experience, you may become more attached to some of the other distributions, as they attempt to target niche audiences. Just as an example, Mythdora is a Fedora based respin (very much like CentOS respins RHEL), and they target users who want to use a computer as their DVR. All the software that comes with Mythdora is open source, and you could install MythTV on any distro, but Mythdora is sort of a package deal, that has it's own following, and community support.
In the end, just being exposed to a new operating system teaches you so much about the way computers work in general. The more you use multiple operating systems, the more you can discover basic principles, and expose yourself to the technology itself, instead of just the pretty lights you see on the display.
It is unfortunate that Linux is compared directly to Windows. When most people say "Windows" they are generally referring to the latest version of Windows, or the version that they are running themselves. Most people are off XP, and Vista and 7 are similar enough they are easily lumped together, so it's really not THAT big of a deal.
However, when you say "Linux", nobody really ever means the same thing. One person's experience with a one distribution will typically cause them to feel the same way towards all Linux distros, when they are very very very different.
Let me see if I can shed a little light on the subject, with my personal experience.
RedHat Enterprise Linux (aka Red Hat, RHEL) is an Enterprise class Operating system, that you must pay for a support license from Redhat to be able to use. Since their Operating system, and much of their software is Open Source, the CentOS project is aimed at stripping all non-GPL content from RHEL, replacing it with their own branding, and building all the same RPMs. CentOS is really only run by a handful of people, who work purely on a volunteer basis. Redhat is mildly interested in the success of CentOS, and don't consider them a threat at all. Redhat is primarily a support company, not a software vendor.
Debian is also an Enterprise class Operating system, based more on BSD principles, rather than SysV like Redhat is. They follow the same types of trends as Redhat as far as release cycles go. They are also on Version 5, update 0. The version numbers between RedHat and Debian are completely unrelated.
Then there's the more desktop centric OS versions. Fedora is Redhat's desktop OS, but it is targeted at the System Admins of their RHEL products. Fedora is sort of a sneak peak at what RHEL will someday look like. They try to have the most bleeding edge version of whatever open source software may possibly have an impact on the RHEL platform. Fedora is not for the faint of heart. It is often so far ahead of the curve, support can be sparse. For example, not many people run a version of the kernel that is less than 2-3 months old. New features in that kernel aren't widely tested, but that's what Fedora is for!
Ubuntu is targeted as as Windows Replacement Desktop. I highly suggest the Ubuntu experience for any one interested in replacing their Desktop OS with a Linux distro. Ubuntu and Debian are both owned by Canonical. Canonical uses their Ubuntu Server distro very much like Redhat uses Fedora.
As far as the big dogs go, Ubuntu is where you start as a desktop user. If you're interested in making a career out of running Linux Systems, both Debian and RHEL are perfectly viable options, but Redhat seems to be more popular in the business place. As you learn more about Linux through use and experience, you may become more attached to some of the other distributions, as they attempt to target niche audiences. Just as an example, Mythdora is a Fedora based respin (very much like CentOS respins RHEL), and they target users who want to use a computer as their DVR. All the software that comes with Mythdora is open source, and you could install MythTV on any distro, but Mythdora is sort of a package deal, that has it's own following, and community support.
In the end, just being exposed to a new operating system teaches you so much about the way computers work in general. The more you use multiple operating systems, the more you can discover basic principles, and expose yourself to the technology itself, instead of just the pretty lights you see on the display.