Edit in JSFiddle

/* Jquery Introduction */
Hello.innerText = $; // init a function

$(document).ready(function() {
  console.log("... The DOM is ready ...");
  // runs when the DOM is ready
  Hello.innerText += "\nMore inner text";
  // selectors derived from CSS selectors
  $('#Hello').addClass("green");
  $('td').addClass("green");
  // selectors and css
  $('td:even').css({
    "background-color": "red",
    "font-size": "2em"
  });
  // and so on ...
  $("td:last-child").html("<h1>Abrahadabra</h1>").css("color","black");
});

<div id="Hello">
  Hello
</div>

<table>
  <tr>
    <td>One</td>
    <td>Two</td>
    <td>Three</td>
  </tr>
</table>

.green {
  background-color: darkgreen;
  color: #fff;
  padding: 1.2em;
}