Originally mainframe architectures relied on a centralized host server that processed data and returned it to be displayed on a dummy terminal. These dummy terminals did not have my processing power and could only display data that was sent from the mainframe.
Application architecture completely changed with the advent of N-Tier architecture. The N-Tier architecture replaced the dummy terminals with standard PCs that could think and/or process for themselves. This allowed for applications to be decentralized. Further, this type of architecture also breaks up the roles found within a mainframe by extracting Web Interfaces, Application Logic and Data access in to 3 separate parts so that it can be extended and distributed as the demands of an application increases.