External

Defined in separate file.

/* path/foo.css */

div {
  background-color: blue;
}

<!-- index.html -->

<head>
  <link rel="stylesheet" href="path/foo.css" />
</head>

Internal

Defined in your html file.

<head>
  <style>
    div {
      background-color: blue;
    }
  </style>
</head>

Defined within the element properties.

<body>
  <div style="background-color: #F55;">
    foo
  </div>
</body>