Explain the Physical Structure of a Database

The SQL data is stored in the database. The data is organized into logical components that are visible to user however data is stored as files on hard disk. Each SQL server has four system databases: master, temp, msdb and model and multiple user databases. How many user databases depend from organization to organization?
The fundamental unit of data storage is page. The page size in SQL is 8KB. Every page contains a page header which is 96 bytes and stores system information such as the type of the page, amount of free space on the page, and object that owns the page.
Extents are another unit which is used to allocate space to pages and indexes. A extent is 8 continuous pages or 64 KB.
SQL server 2k has 3 types of data files: Primary data files which is the starting point of the database and points to other files, secondary data files comprise of data files other than primary data files and thirdly the log files to recover database in the event of a disaster.