Smart Objects, available since CS2, are very similar to linked files.
Smart Objects are layers that contain image data from raster or vector images, such as Photoshop or Illustrator files. Smart Objects preserve an image's source content with all its original characteristics, enabling you to perform nondestructive editing to the layer
In Photoshop CC and CS6, you can embed the contents of an image into a Photoshop document.
Since Photoshop CC you can now also use Linked Smart Objects, which essentially are linked files.
In Photoshop CC, you can also create Linked Smart Objects whose contents are referenced from external image files. The contents of a Linked Smart Object are updated when its source image file changes.
Linked Smart Objects are distinct from duplicated instances of a Smart Object within a Photoshop document. With Linked Smart Objects, you can use a shared source file across multiple Photoshop documents which is a familiar and welcome concept for web designers.