Where g:, b:, t: and w: variables are described?

I recently found that there exists `g:' (without any text after `:') variable
which is a dictionary that holds all other global variables, same for `b:', `t:'
and `w:' variables. Can anybody point me in which help topic they are described?

By the way, the following hack works:
let g:["000"]="test"
echo g:000