API Question

[trekiej]

I understand that an API is the way a program talks to an OS.  Talks may not be the right term.  
So how does an API connect my app. to my OS?
Is there ports or internal functions the API link?
Is there a web site that can help?
Thanks.

[trekiej]

@ Zac67 Wikipedia is where I got my first info. and thanks.

@ Moto

In my opinion a Library works similar to a function.  I would like to know on a low level how an application that I write is able to Interface with the OS.  I believe I understand what an API does but how does it?  If an app. needs to access the disk and requests it through the Kernel how does it request it.
As far as which OS, Linux is one of the targets.
I believe there has to be a way for it to communicate.
Thanks.

[trekiej]

@ Piru
http://en.wikipedia.org/wiki/System_call
I did see Wikipedia earlier and it helps.

[trekiej]

@ bloodline
I am looking for a generalization and am seeing system calls being the conection I am looking for.
It is at least part of it.
edit 15:47
Looking at Linux System Calls I see it goes even deeper than I thought.