j2me
java 2 micro edition APi is used for developing application for small devices like java enabled mobile phones. As these are the small devices which have limitations regarding memory and persistent staorage. So sun has developed a separate API which is suitable for developing application for these small devices.
As per the use of this API is concerned it depends on your requirements only, A large no of developers are working making games for these small devices using j2me and some customized API's given by different mobile phone vendors.
Beside this some people are working on the applications which are for entertainment of end users and also causes to use air time of mobile service providers in this way everytime whenever user will run the application he will enjoy but service provider will earn by the same application.
Although it is a small picture only as the scope or j2me application is more wide then this and can't be explained in few lines only.
Rishi