You separate classes by a space when you want to refer to descendant element and you concatenate them when you want to refer to a single element with multiple classes.
For example, to refer to a div with two classes, e.g. <div class="foo bar"> you could use:
div.foo.bar {...}
To refer to the child span element <div class="foo"><span class="bar">stuff</span></div> you could use:
div.foo .bar {...}