Legislation can be a proposed or enacted law. Law is enacted by a legislative or other governing body.

You can say law is a rule or a set of rules enforced by a superior power or controlling authority, which applies to a group, a whole community or country. Whereas legislation usually means the process of making laws, in other words, it is when someone puts a new rule on paper in order to propose it to the authorities, who will then consider whether to pass it or not, so it can finally become a law.