What is @@ERROR?

The @@ERROR automatic variable returns the error code of the last Transact-SQL statement. If there was no error, @@ERROR returns zero. Because @@ERROR is reset after each Transact-SQL statement,it must be saved to a variable if it is needed to process it further after checking it.

What is SQL server agent?

SQL Server agent plays an important role in the day-to-day tasks of a database administrator (DBA). It is often overlooked as one of the main tools for SQL Server management. Its purpose is to ease the implementation of tasks for the DBA, with its full-function scheduling engine, which allows you to schedule your own jobs and scripts.

Which command using Query Analyzer will give you the version of SQL server and operating system?

SELECT SERVERPROPERTY('productversion'),
SERVERPROPERTY('productlevel'),
SERVERPROPERTY('edition')

Where are SQL server users names and passwords are stored in sql server?

They get stored in master db in the sysxlogins table

What are the authentication modes in SQL Server? How can it be changed?

Windows mode and mixed mode (SQL & Windows).

To change authentication mode in SQL Server click Start, Programs, Microsoft SQL Server and click SQL Enterprise Manager to run SQL Enterprise Manager from the Microsoft SQL Server program group.Select the server then from the Tools menu select SQL Server Configuration Properties, and choose the Security page.