A PSD file is known as a Photoshop document and was developed by Adobe in order for Graphic Designers to create layered images. PSD is the propiertry format that Adobe uses to allow these files to be saved in, they can then be either opened in various versions of Adobe Photoshop or if a user wants to send them to others they can be flattened to PNG, JPG or other more standard image formats. You can't however re-convert a JPG/PNG file that has been converted from a PSD back to a PSD file - it loses it's layers.
Photoshop files have the .PSD file extension, this PSD stores an image with support for a lot of imaging options such as Masks, transparency, clipping paths and duotone settings. Sometimes Photoshop files have the extension .PSB which essentially stands for Photoshop Big, this allows images to support up to a maximum height and width of 300,000 pixels and a length limit of 4 Exabytes.
PNG pronounced ping is a format for storing bitmapped (raster) images on computers. It was created originally to replace the GIF file format when it was announced that the companies who created the GIF format wanted royalties. PNG uses indexed colors and lossless compression (like a .GIF file), but without those copyright limitations; it cannot be animated like a GIF image.
PNG images are in many ways better than .GIF as they also include an 8-bit transparency channel, which allows the colors in the image to fade from opaque to transparent; GIF images only support fully opaque or fully transparent pixels. PNG supports palettes of 24-bit RGB colors, RGB images or greyscale or RGB images. PNG was intended to be able to transfer images on the Internet, not professional graphics, and so does not support other colour spaces (such as CMYK).
Apple Preview Corel Paint Shop Pro GIMP - The GNU Image Manipulation Program (LINUX) Microsoft Windows Photo Gallery Viewer Safari