I saw it online and i dont seem to understand the real meaning of the abbrv API.
Im a young aspiring programmer, so i'll Love to have experts to explain the concept for me.

-the meaning of API.
-how it works
-and it relationship with SMS and BuLk SMS websites.

Thanks as you want to help me.

API is simply a means whereby two applications or websites can share data or talk to each other.

NOTE: One of them have an interface for the other to conect and communicate with.

API Mean : Application Programming Interface,it allow your program to communicate with another program to achieve a task

Depending on the context or application. API may mean the set of command through which a programmer may communicate with another program.

Searches should include Wikipedia, which is surprisingly good for a number of programming concepts/terms such as Application Programming Interface:

What is an API?

An application programming interface (API) is a particular set of rules ('code') and specifications that software programs can follow to communicate with each other. It serves as an interface between different software programs and facilitates their interaction, similar to the way the user interface facilitates interaction between humans and computers.
How is it used?

The same way any set of rules are used.

When and where is it used?

Depends upon realm and API, naturally. Consider these:

The x86 (IA-32) Instruction Set (very useful ;-)
A BIOS interrupt call
OpenGL which is often exposed as a C library
Core Windows system calls: WinAPI
The Classes and Methods in Ruby's core library
The Document Object Model exposed by browsers to JavaScript
Web services, such as those provided by Facebook's Graph API
An implementation of a protocol such as JNI in Java
Happy coding.