What do commas and spaces in multiple classes mean in CSS?

.container_12 .grid_6,
.container_16 .grid_8 {
    width: 460px;
}

That says "make all .grid_6's within .container_12's and all .grid_8's within .container_16's 460 pixels wide." So both of the following will render the same:

<div class="container_12">
  <div class="grid_6">460px Wide</div>
</div>
<div class="container_16">
  <div class="grid_8">460px Wide</div>
</div>

As for the commas, it's applying one rule to multiple classes, like this.

.blueCheese, .blueBike {
  color:blue;
}

It's functionally equivalent to:

.blueCheese { color:blue }
.blueBike { color:blue }

But cuts down on verbosity.

.container_12 .grid_6 { ... }

This rule matches a DOM node with class container_12 that has a descendant (not necessarily a child) with class grid_6, and applies the CSS rules to the DOM element with class grid_6.

.container_12 > .grid_6 { ... }

Putting > between them says that the grid_6 node must be a direct child of the node with class container_12.

.container_12, .grid_6 { ... }

A comma, as others have stated, is a way to apply rules to many different nodes at one time. In this case, the rules apply to any node with either a class of container_12 or grid_6.