There are mainly two type of indexes in SQL, Clustered index and non clustered index. The differences between these two indexes is very important from SQL performance perspective.
1) One table can have only one clustered index but it can have many non clustered index. (approximately 250).
2) clustered index determines how data is stored physically in table. Actually clustered index stores data in cluster, related data is stored together so it makes simple to retrieve data.
3) reading from a clustered index is much faster than reading from non clustered index from the same table.
4) clustered index sort and store data rows in the table or view based on their key value, while non cluster have a structure separate from the data row.