They look similar on a product page and solve different problems. Buying the wrong one is one of the more common desk purchases people repeat.
A laptop stand lifts a screen you type on
It raises the laptop display toward eye level. The moment you do that, the built-in keyboard becomes unusable at a comfortable angle — so a laptop stand almost always means buying an external keyboard and mouse too. Budget for all three or the stand will come off the desk within a fortnight.
Angled stands also help airflow underneath, which matters if the machine runs hot under sustained load.
A monitor riser lifts a screen you already look at, and gives back the space underneath
A riser is a shelf. It raises an external monitor a few centimetres and creates a slot beneath it for a keyboard, a notebook or the clutter that otherwise lives on the surface. It does not change your typing position, because you were never typing on the monitor.
If your problem is "my desk is cluttered", the riser helps. If your problem is "my neck aches by the afternoon", the riser helps only if the screen was genuinely too low.
Measure before either
Two numbers decide this. Desk depth: under about 60 cm, a monitor on a riser can end up too close to your face, which is its own kind of eye strain. Seated eye height: the top of the screen should sit roughly level with your eyes, so measure the gap you actually need rather than buying a fixed height and hoping.
A third option people forget
If the monitor has a VESA mount, an arm does what a riser does and more, and it frees the whole surface instead of part of it. It costs more and needs a desk edge it can clamp to. Check the thickness of your desk before ordering — the clamp range is a specification worth reading.