Although people often say Sass as the name of this CSS-preprocessor, they often mean the SCSS-syntax. Sass uses the .sass file extension, while SCSS-Sass uses the .scss extension. They are both referred to as "Sass".

Speaking generally, the SCSS-syntax is more commonly used. SCSS looks like regular CSS with more capabilities, whereas Sass looks quite different to regular CSS. Both syntaxes have the same abilities.

Syntax

The main differences are that Sass doesn't use curly brackets or semicolons, where SCSS does. Sass is also whitespace-sensitive, meaning you have to indent correctly. In SCSS, you can format and indent your rules as you please.

SCSS:

// nesting in SCSS
.parent {
  margin-top: 1rem;

  .child {
    float: left;
    background: blue;
  }
}

SASS:

// nesting in Sass
.parent
  margin-top: 1rem

  .child
    float: left
    background: blue

After compilation, both will produce the same following CSS:

.parent {
  margin-top: 1rem;
}
.parent .child {
  float: left;
  background: blue;
}

Sass tends to be the more "lazy" syntax. Nothing illustrates this nicer than how you define and include mixins.

Defining a mixin

= is how you define a mixin in Sass, @mixin in SCSS.

// SCSS
@mixin size($x: 10rem, $y: 20rem) {
  width: $x;
  height: $y;
}
  
// Sass
=size($x: 10rem, $y: 20rem)
  width: $x
  height: $y

Including a mixin

+ is how you include in Sass, @include in SCSS.

// SCSS
.element {
  @include size(20rem);
}

// Sass
.element
  +size(20rem)

When it comes to maps, usually SCSS is the easier syntax. Because Sass is indent-based, your maps have to be saved in one line.

// in Sass maps are "unreadable"
$white: ( white-50: rgba(255, 255, 255, .1), white-100: rgba(255, 255, 255, .2), white-200: rgba(255, 255, 255, .3), white-300: rgba(255, 255, 255, .4), white-400: rgba(255, 255, 255, .5), white-500: rgba(255, 255, 255, .6), white-600: rgba(255, 255, 255, .7), white-700: rgba(255, 255, 255, .8), white-800: rgba(255, 255, 255, .9), white-900: rgba(255, 255, 255, 1 )

Because you can format your code on multiple lines with SCSS, you can format your maps to be more readable.

// in SCSS maps are more readable
$white: (
  white-50: rgba(255, 255, 255, .1),
  white-100: rgba(255, 255, 255, .2),
  white-200: rgba(255, 255, 255, .3),
  white-300: rgba(255, 255, 255, .4),
  white-400: rgba(255, 255, 255, .5),
  white-500: rgba(255, 255, 255, .6),
  white-600: rgba(255, 255, 255, .7),
  white-700: rgba(255, 255, 255, .8),
  white-800: rgba(255, 255, 255, .9),
  white-900: rgba(255, 255, 255, 1)
);

Comments in Sass vs. Scss are largely similar, except when multi-lines are concerned. SASS multi-lines are indentation-sensitive, while SCSS relies on comment terminators.

---

Single-Line Comment

style.scss

// Just this line will be commented!
h1 { color: red; }

style.sass

// Exactly the same as the SCSS Syntax!
h1
  color: red

---

Multi-Line Comment

style.scss

Initiator: /*

Terminator: */

/* This comment takes up
 * two lines.
 */
h1 {
   color: red;
}

This will style h1 elements with the color red.

style.sass

Now, SASS has two initiators, but no respective terminators. Multiline comments in SASS are sensitive to indentation levels.

Initiators: // and /*

// This is starts a comment,
   and will persist until you 
   return to the original indentaton level.
h1
  color: red

This will style h1 elements with the color red.

The same can be done with the /* Initiator:

/* This is starts a comment,
   and will persist until you 
   return to the original indentaton level.
h1
  color: red

So there you have it! The main differences between comments in SCSS and SASS!

• SCSS syntax resembles more like a CSS syntax but SASS syntax is little bit different from SCSS but both produces exactly the same CSS code.
• In SASS we are not ending the style properties with semicolon(;) but in SCSS we are ending the style properties with (;).
• In SCSS we used paranthesis {} to close the style properties but in SASS we don't use paranthesis.
• Indentation is very important in SASS. It will define the nested properties in the class or id of the element.
• In scss we can define multiple variables in single line but in SASS we can't do.

With the release of sass 3.3 and plus version the @if and else conditions syntax got same. we can now use expressions with not only scss but also sass.

sass syntax

@for $i from 1 through 3 {
  .item-#{$i} { width: 2em * $i; }
}

Compiled to

.item-1 {
  width: 2em;
}
.item-2 {
  width: 4em;
}
.item-3 {
  width: 6em;
}

scss syntax

@for $i from 1 through 3 {
  .item-#{$i} { width: 2em * $i; }
}

compiled to

.item-1 {
  width: 2em;
}
.item-2 {
  width: 4em;
}
.item-3 {
  width: 6em;
}