Question

I don’t understand the difference between let mapleader="," and let g:mapleader=",". I know that g: means that it’s a global variable, but I don’t clearly understand the difference. Which one should I use in my .vimrc file?

Answer 1

If the aforementioned statements are both located outside of function
definitions, they have the identical effect of setting a global variable.
However, if the first statement (without the g: prefix) is used in
a function body, it defines a variable local to that function. See
:help internal-variables and especially :helpg In a function:.

Hence, outside function definitions one can access the global map-leader
variable simply as mapleader.