The IBM iSeries was developed in the mid-1980s to address a market for applications where a mainframe was either too large or too expensive. The original systems included IBM System 34/36/38 and the AS/400 systems. iSeries is often referred to as AS/400. These systems were cost-effective and efficient alternatives to the mainframe and required far less ongoing administration and maintenance.
While AS/400 refers to the hardware, OS/400 refers to the operating system for iSeries. OS/400 is an operating system where the data, applications, scripting, and communications work tightly together. This makes OS/400 very efficient, but this also can make it difficult to migrate the applications without transforming the entire environment (apps, data, administration).