Vimscript : What is the difference between let g: , let b: , etc.?

I often see in vim plugin something like these :

let g:variable
let b:variable
let l:variable

I made a long research on the vim documentation and on the Internet about these letters 'g', 'b', 'l', but I found noting.

So what is these letters corresponding to ? And what is the complete list of letters ?

See :help internal-variables

It lists the following types:

(nothing) In a function: local to a function; otherwise: global
buffer-variable    b:     Local to the current buffer.

window-variable    w:     Local to the current window.

tabpage-variable   t:     Local to the current tab page.

global-variable    g:     Global.

local-variable     l:     Local to a function.

script-variable    s:     Local to a :source'ed Vim script.

function-argument  a:     Function argument (only inside a function).

vim-variable       v:     Global, predefined by Vim.

b: local to the current buffer

l: local to a function

g: global

:help internal-variables