An index is a small table having only two columns. The first column contains a copy of the primary or candidate key of a table and the second column contains a set of pointers holding the address of the disk block where that particular key value can be found.
The advantage of using index lies in the fact is that index makes search operation perform very fast. Suppose a table has a several rows of data, each row is 20 bytes wide. If you want to search for the record number 100, the management system must thoroughly read each and every row and after reading 99×20 = 1980 bytes it will find record number 100. If we have an index, the management system starts to search for record number 100 not from the table, but from the index. The index, containing only two columns, may be just 4 bytes wide in each of its rows. After reading only 99×4 = 396 bytes of data from the index the management system finds an entry for record number 100, reads the address of the disk block where record number 100 is stored and directly points at the record in the physical storage device. The result is a much quicker access to the record (a speed advantage of 1980:396).
The only minor disadvantage of using index is that it takes up a little more space than the main table. Additionally, index needs to be updated periodically for insertion or deletion of records in the main table. However, the advantages are so huge that these disadvantages can be considered negligible.