An embedded controller is usually a small printed circuit board (PCB) that has a processor of some sort. More importantly, an embedded controller has some form of input/output (I/O) that can interface with electronic or electrical devices.
As you can imagine, embedded controllers are very useful in industry. For example, the engine control unit (ECU) of a vehicle is a specialized embedded controller. Even toasters now have embedded controller to control the heating elements for optimal toasting.
As the electronic devices used on an embedded controller become less expensive, embedded controllers will find their way into more applications. Because embedded applications are diverse, it means that embedded system development job opportunities should increase over time. Furthermore, embedded system development requires both an understanding of software, hardware and even physics, it means that the few who have all the necessary talents will have a lot of options.