The Compute Module datasheet is available here.
In simple terms the RPi Compute Module is a Raspberry Pi 3 without PSU etc circuitry, connectors and with a 4GB Flash IC instead of the SD card socket.
On the PCB:
BCM2837 processor with 1Gbyte of RAM (as used on the Raspberry Pi 3)
4Gbyte eMMC Flash IC, instead of the SD card used on the Raspberry Pi (when comparing costs bear in mind this is an extra cost for the RPi).
All remaining processor interfaces are available to the user via the connector pins, so many more GPIOs and interfaces are available compared to the Raspberry Pi 3.