Most of the activity of the database server is related to queries. Because of this, query optimization is often a major concern. SQL Server tries to optimize query performance itself, but its effectiveness in this may be influenced by several problems. Indexes and poorly designed queries can lead to unacceptable performance, no matter how well you design your database. Key issues related to query optimization include designing indexes, statistics, and index fragmentation. Statistics describe index key values, are maintained for index columns, and are used by SQL Server to decide on the most appropriate index to use when running queries.
Checkout the first of this four part article series here, which gives you an overview of the tools and techniques for SQL Server Query Optimization.
This article is published on SQL-SERVER-PERFORMANCE.COM.