Latex distributions. What are their main differences?

Yours is a list of free TeX distributions.

1. TeX Live (http://tug.org/texlive)

The better maintained TeX distribution, with tens of developers. The distribution of choice on all operating systems. Some GNU/Linux distributions provide it in their packaging framework (Debian, Fedora, OpenSUSE,…), but these prepackaged versions usually lag behind the basic one.

2. teTeX

A defunct TeX distribution once maintained by Thomas Esser, for Unix systems. Perhaps the first TeX distribution in modern terms. It was based on Web2C like TeX Live, and provided some tar archives to just download and install. On some flavors of Unix one needed to compile the binaries, though.

3. fpTeX

A defunct TeX distribution for Windows.

4. emTeX

A defunct TeX distribution for MS-DOS and OS/2.

5. MiKTeX (http://miktex.org)

A TeX distribution for Windows actively maintained by Christian Schenk. Many Windows users prefer it to TeX Live for ease of installation because it is based on a Windows Wizard. Recently it has also been ported to Linux and macOS.

6. proTeXt

A TeX distribution for Windows, essentially MiKTeX with something added.

7. MacTeX (http://tug.org/mactex)

Essentially the same as TeX Live, but only for Mac OS X. It adds some features to TeX Live for better compatibility with the OS.

8. gwTeX

A defunct TeX distribution for Mac OS X once maintained by Gerben Wierda.

9. OzTeX

A TeX distribution for Mac OS up to version 9 (but still runs on some older versions of Mac OS X). The first free distribution for the Macintosh. Not based on Web2C, the source were translated from WEB Pascal into Modula-2.

10. AmigaTeX and PasTeX

TeX distributions for the Amiga.

What are the differences? Well, disregarding the defunct ones or those that run on obsolete operating systems, there's not much to say. For Unix systems there's essentially only TeX Live.

For Windows one can choose between TeX Live and MiKTeX. The binaries of the latter have some different options from the former; which one to install is essentially a matter of personal preference. Both are based on Web2C.

TeX Live

de-facto reference tex implementation cross platform, maintained by volunteers and distributed annually on dvd by local tex user groups.

teTeX

Unix tex implementation, obsolete, precursor of TeXLive

fpTeX

port of tetex to windows, Obsolete.

MiKTeX

Windows specific TeX complementation by Christian Schenk. Recently it has also been ported to Linux and macOS.

proTeXt

Extended distribution of TeX and related programs, based on MikTeX

MacTeX

Version of TeX Live specifically tailored to Mac OS X.

gwTeX

Gerben Wierda's TeX for Mac OS X based (now) on TeXLive

OzTeX

TeX implementation for Macintosh, Obsolete.

AmigaTeX

TeX implementation for Amiga, Obsolete

PasTeX