Distributions generally try to maintain binary compatibility, so a binary compiled on, say, Ubuntu, should run fine on Fedora, as long as all necessary libraries are installed. Any free and open-source application can be adapted to any modern distro, so even if a distro doesn't maintain binary compatibility, they can still run the same applications. They just need to be compiled separately.
There are so many different ways they can differ I can't really cover them all. Virtually anything in Linux distributions can be replaced. Even the kernel can be swapped out with alternate builds. Usually, the most common changes are the default desktop and desktop applications for desktop distributions.
The Linux kernel. Beyond that, there are numerous alternative implementations of virtually every Linux program.
the kernal is the central part of the Linux operating system and determines how the system works - all distributions of Linux are based on this.
GCC is already for the most part preinstalled in many, if not all, GNU/Linux distributions.
Many Linux distributions have multilingual and internationlization, with native applications achieving full translations. However, not all applications installed by the user may have complete translations, if it exists at all. Therefore I suggest if you have the means to do so, contribute or find someone who can help contribute to the language translation efforts for the programs you use in the language(s) you speak.
SSH1 can technically be used on virtually all distributions, yes. It is not, however, in wide use due to certain security vulnerabilities.
All Linux distros have about the same capabilities as far as applications go. The key to playing any Windows games on Linux is the installation of Wine. Wine is a library that allows most (though certainly not all) Windows programs to be run on Linux.
For desktop computers, Canonical's Ubuntu, Linux Mint and Fedora are all very strong distributors for the operating system while for laptops, Fuduntu is a well known and good distributor for Linux.
Different Linux distributions can use different GUI's some will use Gnome or KDE. Most distributions will let you choose which you want. If you have an older machine you might want to use XFCE or LXDE window managers. If you are looking for a distribution you might want to try Linux Mint it uses all of these Window Managers.
All versions of the Linux kernel since 2.0 support IPv6. Most modern distros have IPv6 enabled, but as there are over 300 different distributions, it is impractical or impossible to list all of them.
Linuxreviews.org is the place to go to read about all things related to Linux. The latest applications are listed on the website with plenty of user reviews underneath.
"Types" of Linux are Called Linux Distributions. Linux by itself is only a kernel, you need more than that for a full system, to get this, distro's were develpoed to include all of this to make a Linux system easier to install. Popular Distro's Are Ubuntu, Debian, Fedora, RHEL, Linux Mint, Arch Linux, OpenSUSE, etc, you can find more at distrowatch