Selector:

Specifies which HTML element(s) to apply the style to as shown below

body {
  backgroud: black;
}

Declaration:

is a property value pair that provides instructions to the browser on how to render the content

body {
  backgroud: black;
}

Declaration Block:

It may contain one or more style rules enclosed in curly brackets ’{}’

body {
  backgroud: black;
}