What is the difference between “.class element” and “element.class”?

element.class selects all <element />s with that class. .class element selects all <element />s that are descendants of elements that have that class.

For example, HTML:

<div class="wrapper">
  <div class="content"></div>
  <div></div>
  <div class="footer"></div>
</div>

For example, CSS:

div.wrapper {
  background-color: white; /* the div with wrapper class will be white */
}

.wrapper div {
  background-color: red;   /* all 3 child divs of wrapper will be red */
}