How to determine operating system in elisp?

The system-type variable:

system-type is a variable defined in `C source code'.
Its value is darwin

Documentation:
Value is symbol indicating type of operating system you are using.
Special values:
  `gnu'         compiled for a GNU Hurd system.
  `gnu/linux'   compiled for a GNU/Linux system.
  `darwin'      compiled for Darwin (GNU-Darwin, Mac OS X, ...).
  `ms-dos'      compiled as an MS-DOS application.
  `windows-nt'  compiled as a native W32 application.
  `cygwin'      compiled using the Cygwin library.
Anything else indicates some sort of Unix system.

For folks newer to elisp, a sample usage:

(if (eq system-type 'darwin)
  ; something for OS X if true
  ; optional something if not
)

I created a simple macro to easily run code depending on the system-type:

(defmacro with-system (type &rest body)
  "Evaluate BODY if `system-type' equals TYPE."
  (declare (indent defun))
  `(when (eq system-type ',type)
     ,@body))

(with-system gnu/linux
  (message "Free as in Beer")
  (message "Free as in Freedom!"))

In a .emacs, there is not only the system-type, but also the window-system variable. This is useful when you want to choose between some x only option, or a terminal, or macos setting.

Now there is also Linux Subsystem for Windows (bash under Windows 10) where system-type is gnu/linux. To detect this system type use:

(if
    (string-match "Microsoft"
         (with-temp-buffer (shell-command "uname -r" t)
                           (goto-char (point-max))
                           (delete-char -1)
                           (buffer-string)))
    (message "Running under Linux subsystem for Windows")
    (message "Not running under Linux subsystem for Windows")
  )

This is mostly already answered, but for those interested, I just tested this on FreeBSD and there the reported value was "berkeley-unix".

There's also (in versions 24-26 at least) system-configuration, if you want to adjust for differences in build system. However, the documentation of this variable does not describe the possible vales that it may contain like the documentation of the system-type variable does.