File Name
These are release versions of the GNU FreeFont fonts.

You probably want to get just one of them, the one with the most recent date,
that matches your needs.

As of the 2012 release,

The "otf" files are OpenType, for use in most modern operating systems.

The "ttf" files are TrueType, (strongly) preferred for Windows.

The "woff" files are Web Open Font Format files, for use as Web fonts.

The "src" and "sfd" files hold FontForge "Spline Font Database" files,
for use by font developers.

The ".sig" files are an accountability requirement of GNU upload policies.
You probably don't need that.

(In releases prior to 2012, OpenType was preferred for Windows, and TrueType
for other systems. The essential difference is that now the internal format
of the letters in the OpenType files is cubic spline (PostScript) and of the
TrueType is quadratic spline (TrueType).)