What's the difference between comma separated identifiers and space separated?

For example:
[css]
#identifier_1, #identifier_2, #identifier_3 {…}
#identifier_1 #identifier_2 #identifier_3 {…}
[/css]

I’ve seen both methods used. What type of scenarios would either option be best used in?

The comma is used for grouping, when the same rule applies for several selectors. Each selector is completely independent of the others.

#foo, #bar {color:red}

The rule above is completely equivalent to this,

#foo {color:red}
#bar {color:red}

It’s just a more convenient way of writing.

The space is a ‘descendant combinator’ and means that the element matched by the sub-selector to the right of the space is a descendant (child, grandchild, etc.) of the element matched by the sub-selector on the left-hand side of the space.

For instance, regard the following HTML fragment,

<body>
  <div id="foo">
    <p>First paragraph.</p>
  </div>
  <div id="bar">
    <p>Second paragraph.</p>
  </div>
</body>

The following selectors all match the first paragraph,

#foo p {...}
body #foo p {...}

The following selectors match both paragraphs,

div p {...}
body div p {...}
body p {...}

The selector #foo p should be read (from right to left) as: ‘a paragraph which is a descendant of an element whose ID is “foo”.’