Before you start, go and take a look again at the Multi Player Online Games Directory. Its an excellent resource and gives you an idea of the type of games out there ranging from small free games to the massive games like Sony’s Everquest.
The big multiplayer games aren’t web based but they do work across the Internet. They tend to have their own custom written clients, with encrypted protocols. Fast 3d graphics and large teams of customer support staff. I recommend as an excellent blog on this- you’ll find articles and quotes from luminaries such as Richard Bartle who produced the first Mud (its short for Multi User Dungeon), and Professor Edward Castranova; an expert on virtual world economics. Muds also tend to use their own clients, usually based on Telnet. Although these are not web games, I recommend you widen your experience as much as possible. Muds in particular although not the most popular genre still do exist and provide a good model for interactive games.
Breaking into computer game design and programming is as easy as becoming a fulltime commercial game programmer- i.e. its not! But there is nothing to stop you developing your own web game. And MPOGD will be happy I’m sure to list it in their directory. If its Open Source you can set it up and find other developers on Soiurceforge.net.
There are two categories of web game: real-time and turn based. And this dictates the type of technology that drives it. Real-time is more complicate- the server has to support multiple connections at the same time and care has to taken to synchronise actions between clients. If you attack someone and they’ve already moved but your client isn’t keeping up then your game is not going to be popular.