The words are used interchangeably, but they are not the same. Retinoid is the umbrella term for the whole family of vitamin A derivatives. Retinol is one specific, milder member of that family. Every retinol is a retinoid; not every retinoid is retinol.
The strength ladder
The family sits on a ladder of potency. Retinyl esters (such as retinyl palmitate) are the gentlest and weakest. Retinol is the familiar over-the-counter middle. Retinaldehyde sits a step above retinol. Prescription retinoids such as tretinoin (retinoic acid) are the strongest, because they are the active form your skin would otherwise have to convert the others into.
Why conversion matters
Your skin can only use vitamin A as retinoic acid. The milder forms have to be converted, step by step, before they work. Each conversion loses some potency, which is why retinol is gentler than prescription tretinoin, and why a low-percentage retinol is a reasonable place to begin rather than the strongest thing you can find.
Over the counter vs prescription
Retinol and retinaldehyde are available without a prescription. Tretinoin and other prescription retinoids need a clinician, and are the right route for some concerns, but they are not a starting point you should self-escalate to. Stronger is not automatically better; it is more likely to irritate a barrier that has not built tolerance.
Start low, go slow, and let tolerance build. The result comes from consistency over months, not from the strongest formula on week one.
How to start
Begin with a low-strength retinol two or three nights a week, applied to dry skin, with a moisturiser before or after to buffer. Expect some adjustment in the first few weeks. If your skin stings, flushes, or flakes persistently, step the frequency back rather than pushing through. Always pair a retinoid routine with daily sunscreen.