A website contains multiple components of data, which need to be preserved before making any deletion of data. 
Major components of a ASP.NET website or any websites are:

1. Source Code.
2. Database.
3. Stored Procedure scripts in database.
4. Published code

Source Code: It is the main and most valuable item in web development. It allows developer to make changes and upgrade the app or website in future.

Database: Usually all apps or websites use some data, that needed to be stored in Database management systems. It contains the structure of tables, relationship between tables, stored procedures, views, functions, triggers and most importantly data of website..

Stored Procedures Scripts: Most often database backup is taken at regular interval, but when ever websites need to be migrated or deployed in another server. Database does not contain the scripts of Stored procedures, views, triggers and functions. So it is very important to check that you have taken backup of all those scripts.

Important Note: Database is usually backup in files as per the version of database management system being used, it is recommended to generate script of the database with all tables, stored procedures, triggers, view, functions, user so that it is compatible with all versions of DBMS.

Published Code: Published code is the code hosted on production server, it usually contains configuration settings. 

If you are migrating website from one server to another then mail data also needed backup.