An ISO image is an archive file (a.k.a. disk image) of an optical disc using a conventional ISO (International Organization for Standardization) format that is supported by many software vendors. ISO image is an unattached file format (like RAR and ZIP): it merges a series of files into one single file according to a definite format. ISO image files typically have a file extension of .ISO but Mac OS X ISO images often have the extension .CDR.
VirtualBox is a family of powerful x86 virtualization products for enterprise as well as home use. Not only is VirtualBox an extremely feature rich, high performance product for enterprise customers, it is also the only professional solution that is freely available as Open Source Software under the terms of the GNU General Public License (GPL). See "About VirtualBox" for an introduction.