What is the difference between no-download (Flash or JAVA) and download casino software versions?

The no-download or instant play versions allow you to start playing instantly from almost everywhere without any installation at the expense of a slightly lower graphical quality and sound levels.  At times game play might be slower as well. The no download versions often feature fewer choices of games. That said, with the advancement of internet technologies, the no-download softwares grow more and more popular and will probably soon be even better than the download versions. The latter are however still perceived as more stable and secure.