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The main hub around which UECIDE revolves is the Plugin Manager. Plugins are downloadable content which can be used to extend the functionality of UECIDE - in fact the Plugin Manager itself is a plugin.
Plugins come in four main types at the moment. Java module plugins add extra functionality to the IDE itself. Much of the original features of the Arduino IDE are available as plugins. Some have been replaced with completely re-written programs. Probably the most notable is the Serial Monitor - now re-dubbed the Serial Terminal, as it's much more than just a monitor now. It features a proper ANSI terminal with support for ANSI colour and cursor position codes. So much better for making a proper display of data.
Next are boards. These are small collections of files which define the board and how to program it. They are grouped both by family and by the maker of the board, and we now boast support for most of the mainstream boards from most manufacturers - Arduino, chipKIT, Launchpad, Pinguino, etc.
Cores are the main software libraries that define just how you program your board. They include the instructions for the chip for such instructions as pinMode(), digitalWrite(), Serial.begin(), etc. Without these instructions programming a board is much harder. And of course each chip family requires its own core with its own set of instructions. The Plugin Manager allows you to install different cores for use with the different families of chips, and even multiple cores for the same chip family, all at the touch of a button.
Compilers are also plugins. These are the actual programs used to convert your source code into the executable code used by the microcontroller. Different microcontrollers require different compilers, so adding the compiler you need for your board is pretty much automatic now. If you install a core that requires a compiler you don't have installed, the compiler is downloaded and installed for you automatically.