While state statute usually defines these terms, it is generally true that an heir of a deceased person is someone who inherits from the deceased based upon their biological relationship to the deceased. Who qualifies as an heir is defined by state statute. A legatee (or sometimes what is called a beneficiary) is typically someone whom the deceased has mentioned in their will or other papers with a directive that they are to receive certain property when the individual dies. Heirs are related. Legatees and beneficiaries may not be related biologically.
Always make certain you know the definition of any term used in legal documents by your ancestor. Sometimes a layman’s definition is not the same as the legal one.