What is SAP R/1, R/2 and R/3 .. ? Difference between R/1, R/2 and R/3 ... ?

SAP R/1

SAP R/1 was the first version and it is one tier architecture in which three  layers  Presentation, Application and Data base are installed in one system/server'

Server one- Presentation+Application+Database

SAP R/2

SAP R/2 is the mainframe version of software and it is 2  tier architecture