What is css ruleset?

Ruleset identifies selector or selectors and declares style which is to be attached to that selector or selectors.
For example:
  P {text-indent: 10pt} is a CSS rule.
  CSS rulesets consist of two parts: selector [ e.g. P ] and  declaration [ e.g. {text-indent: 10pt}] .
  P {text-indent: 10pt} – CSS rule (ruleset)
  {text-indent: 10pt} – CSS declaration
   text-indent – CSS property