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

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.