Hey @ezginur-ucan 
George explained it really well, but I’ll throw in my own quick take, and a couple of things I’ve learned the hard way running my own store.
How I think of them:
| Thing | What it feels like | When I reach for it |
|---|---|---|
| Modal | “Stop, decide now.”  |
Big commitments—checkout, GDPR, delete-account |
| Popup | “Hey, quick note!”  |
Grow the list, offer a coupon, cross-sell |
| Overlay | “Let me spotlight this.”  |
Guided tours, loading screens, focus on one CTA |
- Modal: The “you can’t keep going until you do this” window. Like when you have to confirm your shipping address before checkout.
- Popup: More of a gentle tap on the shoulder. You can ignore it and keep scrolling, but it’s there trying to get your attention — usually for things like a discount code or newsletter signup.
- Overlay: The dimmed background or screen tint you see behind a modal, or sometimes by itself to highlight something on the page.
3 little things most people don’t talk about:
- Speed matters way more than design. A slow popup feels annoying even if it’s gorgeous. One test I ran showed faster load (under a second) got way more clicks.
- Mobile “mis-taps” are real. I once had so many people rage-click because my close button was too close to other elements. Adding a tiny invisible buffer fixed it.
- Exit intent works on phones too. You can trigger a “wait, here’s 10% off” popup when someone swipes up fast — it’s saved more carts for me than desktop exit intent.
For my store:
- I use modals for important stuff (checkout confirmation, policy updates).
- Popups for offers (but only after someone’s been on the site for a bit).
- Overlays for onboarding — like showing a first-time visitor where to start without blocking everything.
Small, well-timed nudges beat “in your face” stuff every time.