An Add Form is what you get if you use a Show Form Screen Action, or enable the Add Form option on a list screen, or use a Form Button. You can tell when you’re in a native form, as you have the Cancel/Add options at the top of the screen.
And an Edit Screen is what you get if you enable the Edit option on a Details Screen, or use a Show Edit Screen Action. A native edit screen gives you Cancel/Done options.
These are the “out of the box” options that Glide provides for adding and editing data. The thing that both have in common is that any data that is edited/changed inside the form screen isn’t actually committed to the database until the form is submitted.
But there are other ways to add or edit data. You can stick input components on a details screen and allow users to directly change data. And of course there are the Set Column Values and Add Row actions.
When it comes to Custom Forms, @Jeff_Hager gave a very concise explanation here:
yeah, I’d probably use a Custom Form for that.