What is the Windows equivalent of the diff command?

Run this in the CMD shell or batch file:

FC file1 file2

FC can also be used to compare binary files:

FC /B file1 file2

Winmerge has a command line utility that might be worth checking out.

Also, you can use the graphical part of it too depending on what you need.

Well, on Windows I happily run diff and many other of the GNU tools. You can do it with cygwin, but I personally prefer GnuWin32 because it is a much lighter installation experience.

So, my answer is that the Windows equivalent of diff, is none other than diff itself!