I am confused regarding the use of ' ' and '.' in a jQuery function.
When exactly do you use one or the other?

For example,

```
var main = function(){
    $('.article').click(function(){
        $('.article').removeClass('current')
        $('.description').hide();  
        $(this).addClass('current');
        $(this).children('.description').show();
    }
)};

$(document).ready(main);

```

Why is it correct to use `.addClass('current')` and **not** `.addClass('.current')`, 
or `children('.description')` instead of `children('description')`?

Thank you, I couldn't really find the answer or knew how to look for it on Google.