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.

Which TCP/IP port does SQL Server run on? How can it be changed?

SQL Server runs on port 1433. It can be changed from the Network Utility TCP/IP properties –> Port number.both on client and the server.

What is SQL Profiler?

SQL Profiler is a graphical tool that allows system administrators to monitor events in an instance ofMicrosoft SQL Server. You can capture and save data about each event to a file or SQL Server table toanalyze later. For example, you can monitor a production environment to see which stored procedures are hampering performance by executing too slowly.

What is the difference between a HAVING CLAUSE and a WHERE CLAUSE?

Specifies a search condition for a group or an aggregate. HAVING can be used only with the SELECT statement. HAVING is typically used in a GROUP BY clause. When GROUP BY is not used, HAVING behaves like a WHERE clause. Having Clause is basically used only with the GROUP BY function in a query. WHERE Clause is applied to each row before they are part of the GROUP BY function in a query.