The latter case are typically (massive) multiplayer games, whereas the client-side games are typically single-player games. A game played in a browser is often called a browser-based game. With the development of DOM (document object model) Level 2 technologies in the late 1990s (see Internet Explorer 4, and Netscape 4), it became possible to produce games that would run within a browser without the need of 3rd party plugins. These games were written in DHTML (Dynamic HTML), which is a combination of CSS and Javascript. Normally used for drop down menus and simple image rollovers, DHTML can be used to produce the animation effects required for sprite based action games. Ajax games are produced through the coupling of DHTML and PHP.
A variety of languages are used for programming these games, each with their own attributes.
Java was the first language used for web-based animations and games. It is extremely reliable, requiring no downloads by the user. It runs completely within the window. Java programs are also called applets.
Shockwave frequently allows a higher level of visual complexity, incorporating wider styles of animation and imagery. It has been used to capture many styles, such as anime. Shockwave is widely used for 3D browser based games.
Flash allows a much higher level of complexity in online games. It frequently allows players to use a wide variety of weapons and objects, and can supply much more detailed maps and settings.
The best thing about online games is they don not take any space on your computer, they also allow for more competition, and they can grow as the owner of the arcade adds more to it so you don not have to hunt down new games yourself. This is one plus to online games, the other is that if you have a slow time at work, or at a relatives house then you can spend some time online with their computer and play your favorite games.