A single column index is straightforward. You may have heard it compared to the index in the back of a technical book. To find information in the book, say you want to learn more about how DBCC INPUTBUFFER is used, you look up DBCC INPUTBUFFER in the index. The index doesn’t contain the information on DBCC INPUTBUFFER; it has a pointer to the page where the command is described. You turn to that page and read about it. This is a good analogy for a single column, nonclustered index.
In Microsoft SQL Server, you can also create an index that contains more than one column. This is known as a composite index. A good analogy for a composite index is the telephone book.