The entry point in a plain Swift module is the file in the module called main.swift. main.swift is the only file which is allowed to have expressions and statements at the top level (all other Swift files in the module can only contain declarations).
Cocoa Touch uses the @UIApplicationMain attribute on an implementation of UIApplicationDelegate instead of a main.swift file to mark the entry point. Cocoa used to use a minimal main.swift file which simply called NSApplicationMain, but as of Xcode 6.1 uses the @NSApplicationMain attribute on an implementation of NSApplicationDelegate.