A system call is a function.subroutine which interrupts the currently executing program and transfers control to the interrupt routine.The contents of the executing program are saved and after the interrupt routine finishes its function,control is transferred back to the executing program. If we talk in the context of Linux then System calls are called kernel mode.
Whereas API (Application Programming Interface) is a function or a set of functions, objects, protocols or data-structures for the support of application development for developers/programmers. It is actually a kind of function definition which specifies how to make available of a specific service of the system/OS to the other program/process or system. Read more at: http://www.queryhome.com/48442/os-what-is-difference-between-system-call-and-api
RDO is Hierarchy model where as ADO is Object model. ADO can access data from both flat files as well as the data bases. I.e., It is encapsulation of DAO, RDO , OLE that is why we call it as OLE-DB Technology.
User defined functions can be created into the Microsoft spreadsheet. Spreadsheets can be customized according to the user needs by using VB applications. To create visual basic functions useful for spreadsheet you need to click Tools and select Macros where you will need to click on Visual basic editor.