Mainframe, MVS and zOS Discussion

I have been working on IBM Mainframe for some time and I had been programmer for it. I have been asked to look in one of the projects we have which works on AS400. I have no idea about AS400. Can anyone please tell me what is the difference between IBM mainframe and AS400?

This is one of those questions that are EASILY answered by Google; check out a number of the IBM web pages on the AS400 for details.  The AS400 has a different architecture, different operating system, different file system -- basically mostly what is the same is the IBM name.

Thanks. I did search but I was asking from architecture point of view...

Google is your friend.  Googling as 400 architecture returned over 18 million hits and several on the first page give details about the AS 400 and its successors.  Googling z/architecture returned over 4 million hits and again several on the first page give details about the architecture.  You need to replicate these results and start reading.  Asking a question about differences between architectures is like asking "How high is up?" - the answer largely depends upon how many details you want to get into.