You are welcome Tuskin. Well 'online games' is quite a huge subject. It includes many aspects, I thought up of a different one.
Online games such as Unreal Tour., Quake 3, Half-Life etc. are made in C++ but requires a lot of programming. There is usually a team of programmers who write different modules and classes in C++ and they integrate them at the end.

The info transferred between the client and server sides is quite less. The info is usually in text form so that the game play over the Internet is smooth and fast. The server also takes into account the fast or slow recipients as there are many players belonging different networks.