Depends on the app. Adobe Indesign writes a "file lock" file in the same directory as the InDesign document being edited; other InDesign instances look for that file. Photoshop opens the file, reads it into memory, then closes it. Someone else can open it; if another person saves it, you'll get a message saying "The original file has changed on disk" when you go to save. Quark opens the file read/write and then holds it open; since the operating system allows only one program to open a file read/write, anyone else who tries to open it gets a "This file is already open" error message. Filemaker does record-level locking; two people can edit the same file at the same time but can't edit the same record at the same time.
