The IBM i is a proprietary platform from IBM, which runs on their Power Systems and Pure Systems, and contains a DB2 database integrated into the IBM i OS.
Originally introduced on June 21, 1988 the platform has been continually updated to support and integrate with modern technologies while maintaining source and binary compatibility.
The platform includes an integrated db2 database server and natively supports many application servers including websphere, apache httpd and tomcat.
The proprietary OS uses an object-based architecture, which encapsulates and protects the integrity of objects. An object can only be used in ways allowed for that object type. For example, you cannot open a program as if it were a file for I/O, nor can you execute a file as if it were a program. This is unlike the Unix/Linux/Windows model where everything is a file.
Common programming languages for the IBM i platform include:
The platform is capable of running AIX executables via its Portable Application Solutions Environment (PASE) functionality. This has, in particular, permitted the extensive availability of open source software on the platform (PHP, Python, Node.js, ...). As of v7.2 of the operating system, yum is the supported package manager for most open source solutions that have been ported to IBM i.
For more information see the Wikipedia article on IBM i.
