$1, $2, $3, ... are the positional parameters.
"$@"  is an array-like construct of all positional parameters, {$1, $2, $3 ...}.
"$*"  is the IFS expansion of all positional parameters, $1 $2 $3 ....
$#    is the number of positional parameters.
$-    current options set for the shell.
$$    pid of the current shell (not subshell).
$_    most recent parameter (or the abs path of the command to start the current shell immediately after startup).
$IFS  is the (input) field separator.
$?    is the most recent foreground pipeline exit status.
$!    is the PID of the most recent background command.
$0    is the name of the shell or shell script.