Why we need or use LUA in our application?

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C++

Lua

Hi,
Everybody might be heard about LUA and keeping an understanding and some idea within it.
But, I don't get it rightly.
Why we use LUA in our codes?

All I know in LUA is :

Lua is a powerful light-weight, small footprint programming language designed for extending applications. Along with this Lua provides its own memory management through garbage collection (which can be specialized for your application). Lua is also fully re-entrant code with no global variables.

I already aware that LUA could let us compile variable without even compile entire code. But, how to use it? How do I get there?
The main things that I need to be clear are these :

- What's LUA?
- Why do we need it?
- How to bind LUA and C++?
- Some simple sample (maybe calculator application with LUA and C++ would be much helpful)

So, if anyone over there read and hear this message and have something to enlight me, please reply..

Thanks in advances