These are files containing the installer and other software for the Debian GNU/Linux operating system. The files in this directory are designed to work on both 32-bit and 64-bit PCs (i386 and amd64).
The files here are in "jigdo" format. Use jigdo tools to download the contents of the complete ISO image files from what's here and a normal Debian mirror. You should end up with an exact copy of each ISO image as though you'd downloaded it directly via HTTP or FTP.
Once you have downloaded all the ISO images you want, you will typically need to write them to installation media.
The images described here are sized to be written to writeable CD media at a minimum, but may be written to larger media if needed. For extra convenience, these images may also be written directly to a USB stick. So long as your computer will boot directly from that USB stick, it should start the Debian installer that way.
netinst CD here is a small CD image that
contains just the core Debian installer code and a small core set
of text-mode programs (known as "standard" in Debian). To install
a desktop or other common software, you'll also need either an
Internet connection or some other Debian CD/DVD images.
There are files here (SHA1SUMS, SHA256SUMS, etc.) which contain checksums of the images. These checksum files are also signed - see SHA1SUMS.sign, SHA256SUMS.sign, etc. Once you've downloaded an image, you can check:
For more information about how to do these steps, read the verification guide.
See the Debian CD FAQ for lots more information about Debian CDs and installation.