Designing stand-alone microcontroller based components
A development board typically consists of a microcontroller chip, power supply, pads for the micro controller pins, and peripherals such as USB, LEDs, RAM, FLASH, EEPROM, real time clock, SD card reader, etc. Different boards have different things, depending on the intended application.

Much of the development happens using the development board. Its purpose is to provide an easy environment to get your project started. You may solder a few peripherals on that you need.

Once the project is sufficiently working, the next step is often to custom design a PCB with just the microcontroller and peripherals you need. The code will require minimal adjustment for things such as pin connections changing.

E.g. This pic shows a project I developed using a seeeduino stalker (ATMega328P, real time clock, sd card reader, LiPo charger, XBee slot) and prototype board with an accelerometer I soldered on (a peripheral not on the stalker).



This next pic shows the final version. I only needed the ATMega328P, real time clock, SD card and accelerometer. I changed the power supply to a boost converter so I could use AA batteries. I changed the PCB to a long and thin shape to fit inside the enclosure I wanted. The difference in code between the two is minimal - all that changed was a few pins.



This version of the board is (almost) the one that will go into production. It is actually a bit more expensive than the development board for small runs!