SAP has large number of modules and applications in each and every module.For example if we consider FICO we have many modules like Accounts Payable,Accounts Rceivable,Real Esatate,Treasury etc.So we need to create many invoices in every division and process so much data and need to view reports.For all these we can go either by paths in specified fomat or by transaction codes.Transaction codes are like short cuts to access all the applications.We can access data through tables.