What is the difference between class selectors and id selectors?

Answer

In the CSS, a class selector is a name preceded by a full stop (".") and an ID selector is a name preceded by a hash character ("#"). The difference between an ID and a class is that an ID can be used to identify one element, whereas a class can be used to identify more than one.

#top {
    background-color: #ccc;
    padding: 20px
}

.intro {
    color: red;
    font-weight: bold;
}

<div id="top">
  <h1>Welcome to the CSS3 Tutorial</h1>
  <p class="intro">Select element by class</p>
  <p class="intro">Example for class selector paragraph</p>
</div>