::after

/* Add an arrow after links */
a::after {
  content: "→";
}

/* CSS3 syntax */
::after

/* CSS2 syntax */
:after

<p class="boring-text">Here is some plain old boring text.</p>
<p>Here is some normal text that is neither boring nor exciting.</p>
<p class="exciting-text">Contributing to MDN is easy and fun.
Just hit the edit button to add new live samples, or improve existing samples.</p>

.exciting-text::after {
  content: "<- now this *is* exciting!"; 
  color: green;
}

.boring-text::after {
   content: "<- BORING!";
   color: red;
}

<span class="ribbon">Notice where the orange box is.</span>

.ribbon {
  background-color: #5BC8F7;
}

.ribbon::after {
  content: "Look at this orange box.";
  background-color: #FFBA10;
  border-color: black;
  border-style: dotted;
}

<p>Here is the live example of the above code.<br />
  We have some <span data-descr="collection of words and punctuation">text</span> here with a few
  <span data-descr="small popups which also hide again">tooltips</span>.<br />
  Don't be shy, hover over to take a <span data-descr="not to be taken literally">look</span>.
</p>

span[data-descr] {
  position: relative;
  text-decoration: underline;
  color: #00F;
  cursor: help;
}

span[data-descr]:hover::after {
  content: attr(data-descr
  position: absolute;
  left: 0;
  top: 24px;
  min-width: 200px;
  border: 1px #aaaaaa solid;
  border-radius: 10px;
  background-color: #ffffcc;
  padding: 12px;
  color: #000000;
  font-size: 14px;
  z-index: 1;
}