Clustered Index and Non-Clustered Index

Clustered Index Clustered Indexes are primary keys and uniquely identify the data, they are called clustered index because the data is clustered together on the file-system. Clustering the data allows for quick retrieval and a table can only have one clustered index.

Non-Clustered Index A non clustered index is an index that points to the clustered index. Ideally, the data has limited duplication. For example, take a create date. Rows created on a specific data has a limited scope of that day. Creating a non clustered index on CreateDate will allow quick retrieval of rows grouped by CreateDate. Thing of it as a grouping of all the same data with a reference(clustered index) to the physical row.

Nothing is free, the benefit of creating indexes is speed. Read speed greatly increases. The cost is insert, update and deletes slow down.