By Element

body {
    color: blue;
}

By Class

<p class="abc">ABC</p>

.abc {
    color: green;
}

By id (trumps class)

<p id="xyz">XYZ</p>

#xyz{
    color: red;
}

By Name (or other attribute)

<input name="address1" type="text">

input[name="address1"] {
  color: yellow;
}