Please help understand OOP in Codeigniter

[eluser]mr4k[/eluser]
Hello

I`m new to OOP and learned to call any method inside of the object this way:

Code:

...
$this->method_name();
...

But in CodeIgniter we use more elements (in method calling chain) i cant understand.
Here is a simple line:

Code:

$this->db->where();

"where()" is a method, but what is "db"? ...a method also? ...or an object?
And if it is an object is it safe to use such a way of programming outside the Codeigniter?

I`ve already read bunch of articles about OOP in PHP, but still confused about this issue.

Thank you