Are they synonymous? I don't quite see the difference between them.
- What are some of the relativizers which are not relative pronouns (if any)?
- What are some of the relative pronouns which are not relativizers (if any)?
They're only synonymous if all relativizers (or more simply, "relative words") are pronouns. In English, some belong to other parts of speech.
When and where are often considered relative adverbs, not pronouns, in traditional grammar. If you follow Jespersen and permit intransitive prepositions in your grammar, then these are more naturally considered relative prepositions.
What is often a relative pronoun, but sometimes it's a relative determiner (or "determinative" if you use Huddleston & Pullum's terminology).
That is traditionally considered a relative pronoun, but some linguists argue that it's not a relative pronoun at all, but the same marker of subordination found in declarative content clauses marked by that. In Huddleston & Pullum's grammar, that makes it a subordinator rather than a relative pronoun. Whether you consider this a "relativizer" depends on the grammatical framework you're using.