What's the difference between a Modal, Popup, Popover and Lightbox?
All of these UI components are containers/windows that show on top of the content you are currently viewing/interacting with. The different names are based on the attention they deserve, the context you're in, and how you can interact with them.
Alert - These messages need immediate attention. The window/container is usually locked, meaning you can't dismiss the box or continue with what you want to do until you explicitly acknowledge the content, this usually done by clicking a button inside the container. The buttons on this window are usually "OK" or "Cancel". (If you're showing a lot of these it's usually a failure of design because your user's expectations don't match the UI expectations.)
Modal/Dialog - These are for getting work done. (Example: a button that says "Invite Friends" will open such a container with a list friends you can then invite). These let you do more work without having to show all the information on the main screen. These messages are not locked and you can click anywhere to dismiss the container.
Flash Notice/Growl Notification - These notifications have a time associated with them. You can choose to deal with them right away, or if you don't do anything, after a certain time they will dismiss themselves.
Lightbox/Theatres - These are used to enlarge and bring one part of screen to focus. These containers are most commonly used to view images. Usually you are then able to navigate through similar content (the next photo in the gallery) without dismissing and clicking on another thumbnail. These can be locked or not. Depends on if you want people to return the context they were in or to continue on a new context thread.
I don't know where did you come up with this, every "thing" that shows on top of another "thing" (not dropdowns and menus) and is not a native interface "thing" like the browser `alert/prompt/whatever` is a modal window.
@vsync It is valuable to make distinctions between "things". There is a very specific meaning to the word `modal`. Modal means specifically that you are locked (like the Alert). From WikiPedia: "Users must interact with the modal window before they can return to the parent application. ... The opposite of modal is modeless. Modeless windows don't block the main window, so the user can switch their focus between them, treating them as palette windows." The answer uses `modal` incorrectly. Dialogs may be modal (prompt, alert) or modeless (options, preferences, enter or analyze data, etc).
@vsync Futhermore, what is a "non-native" interface? Native in what context? Not defined in the specific application API, library, framework, etc? Then that is a "fault" of the "native" environment for failing to make the distinction. Or it may be an "optimization" by creating one "native" way to create any "UX" style of dialog. As this is UX Stack, User Experience is the environment we care about, and each of these elements is distinct in the way that humans experience them. So we use a different word to communicate each of these specific experiences, which is useful working with other devs.
I think these definitions are fine for when you are in conversation with a particular group and need definitions to discern each - e.g. a UX meeting. However colloquially a lot of these terms are used interchangeably so it might be misleading to claim these as the true definitions. I only say that because I don't think the colloquial use is *wrong*.
You should add 'toast' somewhere in this list of overlay ux components. Probably alongside "Flash Notice/Growl Notification", I'd never heard of those before.
The definition for a pop-over here is incorrect. A pop-over isn't a passive approach to showing information — in fact, it's often used to host a range of actions. See @Meetai.com 's link, which defines a pop-over as "a transient view that shows on a content screen when a user clicks on a control button or within a defined area", for more information.