Conjunctions connect words, phrases, or clauses. Examples include "and", "but", "or", "because", "so", etc.
Relative pronouns introduce relative clauses, which provide more information about a noun. "Who" refers to people. Other relative pronouns include "that", "which", "whom", "whose", and "where".
Here's an example:
* The woman who lives next door is a doctor.
In this sentence, "who" introduces the relative clause "who lives next door", which tells us more about the woman.