The $ command is a short way to call the element command in order
to fetch a single element on the page. It returns an object that with an extended prototype to call
action commands without passing in a selector. However if you still pass in a selector it will look
for that element first and call the action on that element.
Using the wdio testrunner this command is a global variable else it will be located on the browser object instead.
You can chain $ or $$ together in order to walk down the DOM tree.