The smallest unit of database that SQL Server works with is 8Kilobyte (KB). This 8Kilobyte chunk of database is called pages. SQL Server stores data on the disk in pages form. In Memory also SQL Server database manipulates those same 8 Kilobyte (KB) chunks.

Whole record or row must fit within 8KB chunk (page) when data is written to the disk. Sometime a single page can share more than one row. A row cannot span multiple pages. For Example: If an employee table has employee name, address, city, street, house, state, zip, phone then all that combined data must be written in less than a page or 8kilobyte (KB). Few data types like Binary data, LOB of text files and many more which store pointer to the real data can be spread across multiple pages or even in file. All these data are gathered in to the data files on the disk which has either .MDF or .NDF as file extension.

When a SQL (structured query language) is written in the query analyze of SQL Server database then SQL Server's internal query optimizer looks at that query and constructs an execution plan for executing that structured query. SQL Server uses numerous techniques to execute those different queries. Once SQL Server has the plan for execute a particular query, then it executes it and retrieves the needed data information off of the disk. If the data is requested from the client side then the data is sent over the network.

In case of any modification SQL server use modification query (DML) to modify that query. SQL Server firstly modifies the pages of that data in the memory. After the modification has done, it does not write those updated pages (information) directly to the disk, but SQL server makes a copy of the modification query result in a log file called transaction log file. Transactional log file is that file which keeps track of every transaction in the database. Transactional log file has .LDF extension. If SQL sever writes on disk after every modification is fired on the table then it would increase the system load time to write records on to the disk.

Sometimes SQL server database gets corrupted due to some reasons and backup is not updated then whole SQL server database is inaccessible. At that you have to use third party SQL server database recovery software to recover SQL server database.

Resource: http://hubpages.com/hub/Basic-Overviews-of-SQL-Server