CD/DVD Images (ISOs)
An automated system builds CD and DVD images based on the packages in the archive. Its log files are available from http://people.ubuntu.com/~ubuntu-archive/cd-build-logs/, and the set of packages included on them is driven by the seeds.
New builds are usually produced on a daily basis and published at cdimage.ubuntu.com. Officially released images are published at releases.ubuntu.com. Both are available via HTTP, rsync and BitTorrent.
If you test a CD image, be sure to report your results to the development team.
There are two main varieties of images. One type primarily contains packages, and uses the "alternate" installer (e.g., the server ISO and the alternate ISO). The other contains a casper filesystem and uses the Ubiquity installer (e.g., the desktop ISO)
Further details regarding installer development, including source code access, can be found on the InstallerDevelopment page.
Ubiquity is a graphical installation program which, simply speaking, copies a pre-installed ("live") Ubuntu system and reconfigures it for the system's hardware. It is designed to handle common installation scenarios quickly and easily. Details can be found in the Ubiquity README.
Alternate (Debian) Installer
The Debian installer builds up the installed system from scratch using .deb packages. It is designed for flexibility, and supports more complex installation scenarios. A good overview can be found in the debian-installer paper from DebConf 6.