Grammar: Relative Pronouns
The relative pronoun is a way to continue to make reference to a noun (person, place or thing!), without having to say the noun over and over again.
Pronouns allow you to eliminate the repetition of the noun:
Relative pronouns allow you to link the statements about the nouns together into a single sentence.
The relative pronoun "who" refers back to John: who? John, that's who! John is called the "antecedent" of the relative pronoun. The term "antecedent" means "coming before" -- but don't be fooled: sometimes the antecedent comes after the relative pronoun (especially in Latin!). A better way to think about the antecedent is that before the relative pronoun was inserted, the noun was called by its own name (before he was "who" he was "John").
Latin loves the relative pronoun! In fact, Latin sentences will sometimes go on and on and on and on, so long that in English we would have to break it up into separate sentences. The most common relative pronoun in Latin is "qui, quae, quod." Like other pronouns, it declines for gender and number and case. The relative pronoun gets its case from the verb in its clause, but it gets its gender and number from the antecedent.