Pre-compiled versions are distributed from ftp.gnu.org mirrors. Emacs binaries are distributed as zip files, digitally signed by the developer who built them. Generally most users will want the file emacs-24.5-bin-i686-pc-mingw.zip for the 32-bit build, and emacs-24.5-bin-x86_64-w64-mingw32.zip for the 64-bit build. The zip archive contains everything you need to get started.
The latest source is available from ftp.gnu.org mirrors. It is distributed as a compressed tar file, digitally signed by the maintainer who made the release.
The development version of Emacs is available from Savannah, the GNU development site.