AIX, short for Advanced Interactive eXecutive, is a Unix-based operating system.
The OS was created by IBM and is mainly used on corporate servers due to its strong security options.
IBM launched the OS originally in 1986 and it is now in version 7.1.
The OS supports IBM's own 64-bit POWER processor, and the latest version still works on all hardware past POWER4.
Furthermore, AIX can support Linux applications (after it recompiles them) and supports Java.
List of changes in 7.1 (from Wiki):
Support for 256 cores / 1024 threads in a single virtual machine
The ability to run AIX V 5.2 inside of a Workload Partition
A XML profile based system configuration management utility
Support for export of fibre channel adapters to WPARs
VIOS disk support in a WPAR
Cluster Aware AIX
AIX Event infrastructure
Role Based Access Control (RBAC) with domain support for multi-tenant environments
The OS uses a CDE GUI.