As the name suggests, SoC is a complete system on a single chip. It consists of various components/peripherals which are used in computing.
It consists of basic components like RAM (Random Access Memory) also known as Main memory, ROM (Read Only Memory), CPU (Central Processing Unit), internal storage.
It has its own OS (Operating System).
Voltage regulators and power management circuits.
Timing sources such as phase lock loop control systems or oscillators.
It has many interfaces like Wi-Fi, Bluetooth, USB, Firewire, etc.
It has real-time clocks, counter timers and power-on-reset generators.
It also has digital-to-analog converters and analog-to-digital converters.
All of these peripherals are connected together on one small chip.