The contents of each TeX Live release are also available as large compressed tar files on tug.org. They are primarily for the benefit of distro TeX maintainers, since tar files are easier to work with than ISO images. They are not useful as a direct user-level installation source.
The tar files include: texmf (the texmf trees), source (the TL-specific sources), devsource (TL infrastructure tools), bin (the binaries distributed with TL), and extra (miscellaneous other files).
To save bandwidth, please don't download these from tug.org unless you know you need them.
The active TeX Live development sources are available through entirely different methods (and not related to the devsource tarballs, despite the name).
Information about downloading TeX Live as an ISO image is available separately, as well as other ways to acquire TeX Live.