With a normal CSS style declaration, there is a selector, then an opening curly brace, and then any number of property and value pairs, followed by a closing curly brace. For example:

.css {
  display: block;
}

Where Sass differs is that before the closing curly brace, you can nest another rule within. What do I mean by that? Consider this example:

a {
  color: $color7;
  &:hover,&:focus {
    color: $color5;
  }
  &:visited,&:active {
    color: $color4;
  }
}

In the previous code, a color has been set for the link tag using a variable. Then the hover and focus state have been nested within the main anchor style with a different color set. Furthermore, styles for the visited and active state have been nested...