These casinos all offer a no download version which you may play in your web browser, without downloading and installing any software on your computer. In almost all cases, the casino will require you to have the flash plugin installed (which comes with almost all web browsers these days), although a few may require Java. These casinos can be handy for people who, for whatever reason, do not wish to install the casino software, and most can be accessed by Apple Mac users.
The trade-off made by playing at a no download casino is generally a smaller selection of games, and slightly less impressive animations and sound. But for quick, instant gameplay, not to mention saving space on your computer, nothing beats the convenience of a no download casino.