The cursors are in contrast with the relational model which is based on the set Theory .
The cursors are slow as compared to the relational based set implementation as they manipulate and work on the data row by row iteratively which is typically slow.
One of the main reasons of cursors are slow is because of overhead being caused due to : declaration of the cursor , then opening the cursor , looping through the cursor , closing the cursor and then de allocating the cursor .
Use cursors when you have a situation where a not a better solution exists from logical set based implementations .
A good example where you can cursors is to run it to know the total number of rows in a table.
SqlBird 