Despite this, I have never gotten clear on what differentiates different Linuxes. There are different flavors (Debian, Fedora, Arch, CentOS, Gentoo, SUSE,...). These can be the basis for other distributions such as the many Debian-based ones that include the Ubuntu-family (more later), as well as Raspbian, Knoppix, MEPIS, etc. Finally, there are different sub-distributions often characterized by a particular choice of desktop environment. In particular, I'm thinking of the huge number of official and unofficial Ubuntu variants and Ubuntu-based distributions including:
Special interest groups (SIGs) are organized portions of the CentOS community that open paths for building specialized variants of CentOS, which fulfill specific sets of requirements. SIGs have the freedom to modify and enhance CentOS in various ways, including adding more cutting-edge software, rebuilding existing packages depending on the requirements, providing alternative desktop environments, or making CentOS available on otherwise unsupported architectures. Among other things, this provides an opportunity for the community to get the best of both worlds – the overall stability of CentOS and newer technology from various open-source projects. 
Linspire has drawn some criticism from the free software community . This has included criticism for including proprietary software , with GNU founder Richard Stallman commenting: "No other GNU/Linux distribution has backslided so far away from freedom. Switching from MS Windows to Linspire does not bring you to freedom, it just gets you a different master."  In addition, following the initial Freespire announcement Pamela Jones of the Groklaw website published an article entitled "Freespire: A Linux Distro For When You Couldn't Care Less About Freedom;" that was highly critical of Linspire, Inc., and the Freespire project, for including closed-source components and advertising them as a favourable point—an action she classed as ignoring free and open-source software (FOSS) community values in a "community-driven" distribution, asserting that "Free Software isn't about proprietary drivers" and that "proprietary codecs, drivers and applications are not Open Source or open in any way."  In response, Linspire, Inc. CEO Kevin Carmony stated via a journalist on the Linspire website that in ten years of holding out, the FOSS community has made relatively few gains, that many users are already using proprietary software and, although some would hold out, most would prefer to have something that works rather than nothing. He also asserted that the company believed in open source software , but also in the freedom of individuals to choose whatever software they want.