css should i use id or class code example

Example 1: css class and id

Select an element with the ID "id" and the class "class":
#id.class {
}
example:
<div>
  <strong id="id" class="class">
      Foobar
  </strong>
  <strong class="class">
      Foobar
  </strong>
</div>
=> Will select the first <strong> element

Select all elements with the class "class",
which are decendents of a element with an ID of "id":
#id .class {
}
example:
<div id="id">
	<strong class="class">Foobar</strong>
</div>
=> Will select the <strong> element

Example 2: css class id

css
.class
#id