Actually, an AS/400 and and iSeries are really different names for the same thing!  In late 2000, IBM reorganized their product lines and gave them new category names under their newly labeled branch of eServers:
 

Product Line

Previously Known As

iSeries

Mid-range systems (AS/400, System/34/36/38)

pSeries

Unix systems (RISC6000)

xSeries

Intel systems (PCs, Network Servers)

zSeries

Mainframes (43xx, 309x)