A PSD file is a Photoshop document. It is an image file that may contain multiple layers and may be saved in one of several different color modes. PSDs also contain metadata and other information recognized by Photoshop.
About Adobe Photoshop
Photoshop is a popular professional graphics editing program. It is designed primarily for editing raster images, such as digital photos, but also supports vector graphics and 3D images. Photoshop documents may include multiple layers and may be saved in one of many different color modes.