Maruthi,
SAP provides most of the reports that would be required in a business considering that the organization follows the typical standard way of running the business. Customization comes into picture when the organization wants to differ from the standard way or add its own functionality to standard processes, thats when custom reports, programs or enhancements come into picture.
Just an example for report program: business may want to get a mail which has the list of all sales orders created for that day. So, we will have to write a report that will fetch the sales order created from respective tables, put it in a file, attach it to email and send it. The program would be run as a background job every day so that status is sent in mail.
Hope this helps,
~Athreya