ARW format is format developed by company named Sony. Its whole name is Sony Alpha RAW. We can say that it is RAW format, which is modified by Sony for their needs. It is derived from format TIFF. Its advantage is taking high quality pictures that can be easily transferred into another file types like JPG or TIFF. The main disadvantage is huge size, which is cause by its quality.
The easiest way to view ARW files is to reformat them into format JPG. But it will decrease the quality of this file. There are also many programs allowing to open them. Sony on their own created ARW Viewer, which allows to show and partially edit and reformat files with this extension. Program allows conversion to JPG, GIF, PNG, BMP and TIFF. Available is only for Windows XP and newer. So you can use this program called ARW Viewer to view these files. He´s available for free.