open is a Nextstep & Mac OS X command line process that opens file(s), folder(s) or URL(s) in the GUI as though the user had double clicked on them. Files will be opened in the default application for their type, folders will be opened in the Finder or file system GUI, and URLs will be opened in the default browser.
The counterpart of the process in Microsoft Windows is start.exe. In KDE, kfmclient exec has the same effect.