What is it?

$? is a built-in variable that stores the exit status of a command, function, or the script itself.

$? reads the exit status of the last command executed. After a function returns, $? gives the exit status of the last command executed in the function. This is Bash's way of giving functions a "return value." It returns 0 on success or an integer in the range 1 - 255 on error.

Are there others like it too?

Yes,there are several such built-in variables in bash. You can see a list here.
Refer:
http://www.tldp.org/LDP/abs/html/exit-status.html