Unlike Windows, you know that it is from Microsoft and is only one. Unix, however is not a single operating system. It has many flavors (aka. variants, types, or implementations). Although based on a core set of Unix commands, different flavors have their own unique commands and features, and designed to work with different types of hardware.

The following are some of the well-known Unix flavors. By no means this is complete. No one knows exactly how many variants UNIX has, but is surely in the hundreds, including the obsolete ones.

AIX by IBM
BSD/OS (BSDi) by Wind River
CLIX by Intergraph Corp.
Debian GNU/Linux by Software in the Public Interest, Inc.
Tru64 Unix (formerly Digital Unix) by Compaq Computer Corp.
DYNIX/ptx by IBM (formerly by Sequent Computer Systems)
Esix Unix Esix Systems
FreeBSD by FreeBSD Group
GNU Herd by GNU Organization
HAL SPARC64/OS by HAL Computer Systems, Inc.
HP-UX by Hewlett-Packard Company
Irix by Silicon Graphics, Inc.
Linux by several groups several
LynxOS by Lynx Real-Time Systems, Inc.
MacOS X Server by Apple Computer, Inc.
NetBSD by NetBSD Group
NonStop-UX by Compaq Computer Corporation
OpenBSD by OpenBSD Group
OpenLinux by Caldera Systems, Inc.
Openstep by Apple Computer, Inc.
Red Hat Linux by Red Hat Software, Inc.
Reliant Unix by Siemens AG
SCO Unix by The Santa Cruz Operation Inc.
Solaris by Sun Microsystems
SuSE by S.u.S.E., Inc.
UNICOS by Silicon Graphics, Inc.
UTS by UTS Global, LLC