What is J2ME?

J2ME is the short form for Java 2 Micro Edition. J2ME is meant for tiny devices such as mobile phones, TV set top boxes, Vehicle telematics, pagers, PDAs etc.

There are many differences between J2ME and other Editions of Java. The reason being the target devices of J2ME are very much different from the Computers.
Some of the main differences between computers and the J2ME devices are:

Limited processing power
Limited system memory
Limited storage capacity
Small display
Less Battery power
Limited connectivity to internet

J2ME was designed with all these above considerations. The J2ME compliant device manufacturers include the miniature version of the JVM in their devices, which is very light weight and suitable for these small devices. This JVM enables the execution of small Java programs which are called midlets. These midlets since they are nothing but Java programs (a little bit different) make these devices very powerful.
The J2ME applications can be used to do many useful things. A few of the capabilities of a J2ME program are:

Making UDP connections back to the server or communication between two devices.
Making HTTP connections back to a HTTP server to make rich applications
Making Socket connections
Bar Code scanning
Bluetooth programming
and many other programs limited only by your imagination.