There are two types of CSS rules:
(i) Ruleset
Ruleset recognizes selectors and declares style which is to be connected to that selectors. For instance P {text-indent: 10pt} is a CSS rule.
CSS rulesets comprise of two sections: Selector and Declaration
e.g. {text-indent: 10pt}.
P {text-indent: 10pt} – CSS rule (ruleset)
{text-indent: 10pt} – CSS declaration
(ii) At-rule
At-rule is a standard that applies to the entire style sheet and not to a particular selector just (like in ruleset). They all start with the @ symbol pursued by a keyword made up of letters a-z, A-Z, digits 0-9, dashes and escaped characters, for example, @import or @font-face.