In addition to Idriss answer:

CSS

In CSS we write code as depicted bellow, in full length.

body{
 width: 800px;
 color: #ffffff;
}
body content{
 width:750px;
 background:#ffffff;
}

SCSS

In SCSS we can shorten this code using a @mixin so we don’t have to write color and width properties again and again. We can define this through a function, similarly to PHP or other languages.

$color: #ffffff;
$width: 800px;

@mixin body{
 width: $width;
 color: $color;

 content{
  width: $width;
  background:$color;
 }
}

SASS

In SASS however, the whole structure is visually quicker and cleaner than SCSS.

• It is sensitive to white space when you are using copy and paste,

• It seems that it doesn't support inline CSS currently.

  $color: #ffffff
  $width: 800px
  $stack: Helvetica, sans-serif
  
  body
    width: $width
    color: $color
    font: 100% $stack
  
    content
      width: $width
      background:$color
  

Just to give some examples, because that's often a neat way to explain the difference:

CSS (what the browser understands):

h1.foo {
  background: red;
}
h1.foo .bar {
  text-shadow: 0 1px red;
}

SASS (compiles to CSS, supports things like nesting and variables):

$color: red

h1.foo
  background: $color
  .bar
    text-shadow: 0 1px $color

SCSS (SASS, but written with a CSS-like syntax):

$color: red;

h1.foo {
  background: $color;
  .bar {
    text-shadow: 0 1px $color;
  }
}

The latter two compile to CSS when you run them through a program. You then send the compiled files to the browser, since browsers only understand CSS.

Sass is a CSS pre-processor with syntax advancements. Style sheets in the advanced syntax are processed by the program, and turned into regular CSS style sheets. However, they do not extend the CSS standard itself.

CSS variables are supported and can be utilized but not as well as pre-processor variables.

For the difference between SCSS and Sass, this text on the Sass documentation page should answer the question:

The SCSS syntax uses the file extension .scss. With a few small exceptions, it’s a superset of CSS, which means essentially all valid CSS is valid SCSS as well. Because of its similarity to CSS, it’s the easiest syntax to get used to and the most popular.

The indented syntax was Sass’s original syntax, and so it uses the file extension .sass. Because of this extension, it’s sometimes just called “Sass”. The indented syntax supports all the same features as SCSS, but it uses indentation instead of curly braces and semicolons to describe the format of the document.

However, all this works only with the Sass pre-compiler which in the end creates CSS. It is not an extension to the CSS standard itself.