Most of these readings will be from wikipedia - they're great pages to start. If you're still confused, I suggest you do a little more background research to clarify what these concepts are before moving on.

Microcontroller Embedded System I2C UART ADC SPI

1. What is an embedded system?
2. What is a microcontroller?
3. SPI, ADC, UART, I2C
   1. What do they stand for?
   2. What are they?
   3. What are they useful for?
   4. How are they all related?