Three-tier architecture is a well-established software application architecture that organizes applications into three logical and physical computing tiers: the presentation tier, or user interface; the application tier, where data is processed; and the data tier, where the data associated with the application is stored and managed.
UI(Frontend)-->API(Backend)-->Database
Presentation tier (UI): Will have the design of the website that we browse usually that would be designed in HTML or PHP.
Application tier (API) : This will have the actual .net functions that will help UI in processing the data. UI will connects to API to process any logics
Data tier (SQL): This is a database tier where the processed data will be saved in the database for further reference, we usually use SQL server as a database to .Net based applications SQL Server Versions are like this 2000, 2005, 2008, 2008 R2,2012, 2014, 2016 , 2019(Running Version)