Adware refers to programs that display advertising using any means: pop-ups, banners, changes to the browser home page or search page, etc. The advertisements could be associated with the products or services offered by the creator of the program or by third-parties.
Adware can be installed in a number of ways, on some occasions without users' consent, and either with or without users' knowledge of its function.
Adware/AdOnDemand does not spread automatically using its own means. It needs an attacking user's intervention in order to reach the affected computer. The means of transmission used include, among others, floppy disks, CD-ROMs, email messages with attached files, Internet downloads, FTP, IRC channels, peer-to-peer (P2P) file sharing networks, etc.