J2ME can be divided into three parts.
A Configuration: A configuration contains the JVM (not the traditional JVM, but the cut-down version) and some class libraries.
A Profile: a profile builds on top of these base class libraries by providing a useful set of APIs
Optional Packages: An Optional set of APIs that you may or may not use when creating your applications. Optional packages are traditionally not packaged by the device manufacturers, and you have to package and distribute them with your application.
The configuration and profile are supplied by the device manufacturers and they embedded them in the devices.
The most popular profile and configuration that Sun provides are the Mobile Information Device Profile (MIDP) and Connected Limited Device Configuration (CLDC), respectively. As the name suggests, CLDC is for devices with limited configurations; for example, devices that have only 128 to 512KB of memory available for Java applications. Consequently, the JVM that it provides is very limited and supports only a small number of traditional Java classes. (This limited JVM is actually called the KVM.) Its counterpart, the Connected Device Configuration (CDC) is for devices with at least 2MB of memory available and supports a more feature-rich JVM (but still not a standard JVM).
The MID profile complements the CLDC configuration very well because it minimizes both the memory and power required for limited devices. It provides the basic API that is used for creating application for these devices. For example, it provides the javax.microedition.lcdui package that allows us to create the GUI elements that can be shown on a (limited) device running the MID profile on top of a CLDC configuration. Note that MIDP cannot be used with CDC devices. CDC devices get their own set of profiles, like the Foundation and Personal profiles. However, I will not cover these profiles or the CDC here, and will concentrate on using MIDP and CLDC only.
The MIDlet Lifecycle
Mobile devices, whether emulators or real, interact with a MIDlet using their own software, which is called Application Management Software (AMS). The AMS is responsible for initializing, starting, pausing, resuming, and destroying a MIDlet. (Besides these services, AMS may be responsible for installing and removing a MIDlet, as well.) To facilitate this management, a MIDlet can be in one of three states which is controlled via the MIDlet class methods, that every MIDlet extends and overrides. These states are active, paused and destroyed.