Hi Experts,
     I would like to know the exact difference between 15 and 18 digits record id. Most of us say, 15 digit(case sensitive) and 18 digit(case insensitive).

Let's consider a small example,
While if you query record from Developer Console you will get 18 digit record id. Consider the below retrieved 18 digit id,

001i000001AWbWuGTA(Retrieved from query)

If 18 digit case insensitive means,

001i000001AWbWugta (this id same to the above retrieved ID)? If yes, then if you queried record with this id means, you will get no result or no rows found..!

So if both 15 and 18 are unique means what is the real time use and difference between these two ID's.

Thanks in advance..!