These are snapshot builds from various hack branches containing code that may or may not be intended to ever be merged into a main version.
The styct versions are from the popular sty+ct branch and support additional game modes such as CTF and shooting.
The t2o versions are test versions with compatibility to the fork Tr2n Origins. They probably don't work any more.
The school versions are builds for a closed school only network that didn't catch on.
*dedicated*: Dedicated server; everything else is the game client/hybrid server
*.exe: Windows installer
*.dmg: Mac drive image
*.package: Linux Autopackage installer
*.deb: Debian packages; must install armagetronad_common*.deb first.
Armagetron*: Portable Linux Apps for 32 bit systems. Just download, mark as executable and run.
*.src.tar.*: Unix and Mac source tarball.
*.src.zip: Windows source zip. Almost identical to the tarball, but has the Unix build system removed and a couple of files converted to DOS mode.
*.bin.tar.bz2: Binary tarball for use with Zero Install on Linux. Best not to manually download those.
*.win32.zip: Binary zip for use with Zero Install on Windows. Best not to manually download those.