Both are totally different.
<!-- Here you're declaring ONE div element with two values on class atribute -->
<div class="myclass container">some code</div> this format
<!-- Here you're declaring TWO elements with a different class each one -->
<div class="myclass">
<div class="container">
</div>
</div>
Why this is so different?
HTML tags/eelements have default properties with default values, plus the properties and values that you put in addition.
For example:
if you set globally:
div{padding:5px;}
On the first example, the content inside the div will be padded 5px.
On the second example, the content inside container will be padded 10px.
That can happen with default properties rendered by the browser or globally applied by frameworks as bootstrap.