A kludge that tries to adhere to both the BSD and AT&T unix tastes. Also one of my favourite unices. It has borrowed some features from 'real' operating systems, including a virtual machine-ish approach to low-level functions like load balancing between processors, and an internal database for the sub-os/VM level configuration (like the windows registry) rather than config files.
It also uses SMIT, a nice menu-driven configuration tool that builds scripts in Korn shell (AIX' native shell) and lets the adminstrator edit the actions before committing the actions. For a novice AIXer, familiar with other unices, this is extremely helpful since many commands have strange names and args.

Powerful, but not a very clean unix implementation. It tries to provide both BSD and System V command sets by including redundant flags for most commands, but has a number of non-standard replacements for some commands as well.