simple Q a/b '$HOME', etc.

[R1nso13]

i've noticed in a lot of shell scripts for bash items beginning w/ a '$' used 
to refer to things. for example: i'm pretty sure '$1" refers to any arguments 
given to the script. 
so what is this method of referring to things called? and is there somewhere 
i can find a list of all of the "$_____" things out there?