There are Vim and Vimscript documents scattered around the web that I refer to. With the exception of the brilliant http://learnvimscriptthehardway.stevelosh.com/ , the one I've visited the most is http://www.ibm.com/developerworks/library/l-vim-script-1/ - specifically for the table below (just about the only failing of Learn Vimscript the Hard Way is not having an equivalent):

Vimscript Variable Scoping:

Prefix	Meaning
g:varname	The variable is global
s:varname	The variable is local to the current script file
w:varname	The variable is local to the current editor window
t:varname	The variable is local to the current editor tab
b:varname	The variable is local to the current editor buffer
l:varname	The variable is local to the current function
a:varname	The variable is a parameter of the current function
v:varname	The variable is one that Vim predefines

A similar table that I want to keep with it:

Vimscript Pseudovariables:

Prefix	Meaning
&varname	A Vim option (local option if defined, otherwise global)
&l:varname	A local Vim option
&g:varname	A global Vim option
@varname	A Vim register
$varname	An environment variable