OS-tan Class
Classes of OS-tans and hardware-tans, sorted by size of hardware
Mainframe
Includes OSes for mainframe systems.
Examples: ENIAC, Harvard Mark I, PDP-10, Colossus.
Oftentimes, the distinction between mainframe hardware-tans and mainframe OS-tans isn't made because several characters may represent both.
Examples: Cray-1 (also represents Cray OS); SAGE (also represents the AN/FSQ-7 hardware). Other mainframe OS-tans, such as Multics, ITS and CTSS may also represent their OS ran on.
Minicomputer
Relatively small workstation or server-class computers of the 60's and 70's and their OSes.
Examples: DEC PDP line (except for the PDP-10).
Microcomputer
Desktop and laptop computers, and the OSes they run.
Workstations
High-end microcomputers and OSes mainly used for business and scientific applications. There is considerable overlap between workstation and server OSes.
Examples: Xerox Alto, Xerox Star, NeXTSTEP, IRIX, AIX, Solaris, Windows NT, DEC VAX (VMS, RSX-11, VAXELN).
Servers
Another type of high-end microcomputer; computer systems mainly used to store files for a network, and can be accessed by others.
Examples: NeXTSTEP, IRIX, AIX, Solaris, Mac OSX Server, VAX, Windows Server, Windows NT.
Personal Computers
Microcomputer systems mainly for home and small-business use.
"home computer"
A historical grouping the low-cost 8-bit and 16-bit personal computers of the 70's through the early 90's. Many have a built-in programming language interpreter, most commonly BASIC.
Examples: Apple II, Commodores, Atari computers.
Multi-edition OSes
An OS with separate variants for different types of systems and uses. These include personal computer (home or professional), workstation, server and mobile editions.
Examples: Windows XP, Vista, 7 (each with home, professional, enterprise and ultimate editions), Mac OSX (OSX and OSX Server), Ubuntu (Desktop and server editions)
Mobile
Examples: Windows CE, Windows Mobile, Android, iOS.
Other Classes
These classes don't neatly fit into the above size-based categories.
Embedded OS
OSes that may be ran on electronic systems of any size, desktop computers included, but rarely used on desktops. Most commonly dedicated to the system they run on, and many are real-time OSes.
Examples: ROMDOS, OS-9, QNX.
Prototype
Computers or OSes of any class that were never officially released.
Examples: Copland, Windows Odyssey, Windows Neptune, Commodore 65.
Hobbyist (computer or OS)
Hardware or OSes made mainly for non-commercial use, by hobbyists, and mainly for hobbyists.
Examples: MenuetOS, DexOS, DigiComp.