This .NET feature combines the performance of local installation with the convenience of running from the server. The application is made available on a web server or file server; the user clicks a link to run it. The application is copied transparently to an assembly cache and executes from there. It is not downloaded again unless it has been updated. It can also be run offline if this kind of operation is specified. More details here:
Comments: Click-once is not so very different from Java Web Start, so not an unique .NET feature. Another point is that some apps may require more complex installation procedures, which need Windows installer or other setup applications. By the way, No-touch is in .NET 1.0 and 1.1, while Click-once is an enhanced version in .NET 2.0. Potentially it combines some of the advantages of web apps (always up-to-date, no installation) with those of windows apps (rich user interface, executes locally). Obviously you still need to have the .NET runtime deployed, and this is Windows-only technology.