An appositive is a noun or noun phrase that defines or restates another noun (or pronoun). Generally, the appositive follows the word it defines, as in My friend, Susan, came to dinner. But the appositive can also precede the noun it defines. Study this example:
A landmark decision, Brown v. Board of Education desegregated the public schools.
If an appositive is nonrestrictive (most are), it should be set off by commas. But if an appositive is restrictive, no commas should appear. Study these examples:
John's brother Fred came to dinner (restrictive, John has more than one brother and the appositive Fred singles out which brother, no commas).
Susan's brother, Jack, came to dinner (nonrestrictive, Susan has just one brother, commas are mandatory).
The terms restrictive and nonrestrictive are elsewhere defined.
See restrictive clause and nonrestrictive clause.