What does deleting a file really do? When I run an app, select a file, and choose "delete" what happens then:


the file is moved to trash (and later is removed by OS when some conditions are met)
the file is removed from the filesystem at once
(1) or (2) depending on an app


?
There's no OS-wide "Trash" or "Recycle bin" like other shells have. However, the Android design guidelines recommend app authors to put something like that in each app as appropriate, to allow undoing deletes. Thus, in one app, a delete button might delete the content immediately, while another might mark the content as deleted but wait for some time before deleting any files in the filesystem.

There are even more possibilities with apps that sync to a server: they might need to keep 'deleted' content around for the next sync, or it might be on a server somewhere, never deleted.

In addition, not every item of user data you might delete is a file. Some apps might store things in a database, so after you delete an item, part or all of it might still be present in the app's database.

Of course, like any filesystem, once a file is deleted, most of its content remains on the storage. Someone with physical access to the storage, or a rogue root app, can almost certainly still read it for quite some time after it's deleted. It's pretty easy to recover files in this way, especially if your phone acts as a USB mass storage device when plugged into a computer.