What is SOC in embedded System
The term SoC has a bit of a marketing tinge to it.
System on chip (SoC) implies that a single chip contains silicon, which provides hardware support for a lot of functions.  Previously, these functions would require multiple chips (a chip set).

Here's a menu of functionality, which can be found in SoC.
(Again, this is a menu.  I'm not implying that everyone of these features has to be included to a chip to qualify as a SoC.  Neither this menu is complete.)


Computing:  CPU, DSP core, MPEG codec.  Some of the recent SoC have multiple cores.
Memory:  RAM, non volatile memory
Wired communication: USB slave (and master in some cases too), Ethernet MAC (and PHY in some cases too)
Wireless communication:  Bluetooth, WiFi
Embedded buses:   CAN, I2C, SPI
Analog capabilities:  A/D, D/A
Even programmable logic fabric (similar to FPGA or CPLD) is included in the same chip in some cases