When available, this directory contains preliminary test images and checksums for the upcoming TeX Live release. (The current official release is available from CTAN.)
As an alternative to these giant images, for normal testing we recommend the small installer package(s) or other methods, as described on the TeX Live pretest page. That page also contains an overview of changes and links to the documentation.
The TeX Live web pages describe mounting the .iso files and everything else.
The ISO excludes the binaries for less-used platforms, to save space on the DVD. These platforms are in the TL repository and can be installed normally over the net.
The same data is also available as large compressed tar files, primarily for the benefit of distro maintainers: texmf (texmf trees), source (sources for the compiled programs in TL), devsource (TL infrastructure sources), bin (binaries for all platforms), and extra (miscellanous other files). See the directory listing.
The images are updated occasionally, usually finishing by 6am Denmark time. Please don't consume bandwidth retrieving new versions every day without specific reason; changes are rarely that significant. Using another method is far better for daily updates.
If you have problems running TeX after installation, please check your environment variables: any settings that end up referencing other TeX installations (TeX Live or otherwise) usually cause trouble.
Bug reports are especially welcome, since these are test images. Any problems not found in testing will be present in the real release, so please test anything you can. In general, please email tex-live@tug.org with any problems, questions, suggestions. The tex-live mail archives may be of some use.