>Hi,
>I am confused with this what they call it “API”
>

Application Program Interface - a standard mechanism to connect to external services.

>Kindly, Can anyone tell me in short WHEN & WHY we need to use the API and how it’s helpful?
>

When there's a program service which is available to the system, and not native to VFP, it's often possible to access the service through a published interface - the most common ones being COM, used to interact with ActiveX/OLE servers, and the standard C style DLL interface used by most of the operating system functions collectively referred to as the Win32 API.

The odds are that if you aren't aware of when you need it, you probably don't need it. If you don't know why you need it, it's because other programs don't talk directly to the VFP environment; they use a standard interface, or API, to access these services.

>And another favor:
>
>What are the rolls/steps to call any API? Like what I need to know (giving values.. return values … etc..)

There's no single answer to this. Look at the DECLARE...DLL command, which is used to define API interfaces to VFP. There are articles in the FAQ giving some general direction. If you know VB, Dan Appleman's "Visual Basic Programmer's Guide to the Win32 API" is a good reference that does not require a knowledge of C to use; it is not VFP, but the syntaxes for VB and VFP are similar.

There are literally several thousand standard MS-defined API calls that are a part of the Platform SDK; the detailed (C syntax) definitions for them appears in the MSDN Library. Third-party services which use API-style calls also exist, and there's no single source of reference on the third party components.