The Compute Module is a Raspberry Pi 3 in a more flexible form factor, intended for industrial application. CM3L (Compute Module 3 Lite) is the Raspberry Pi 3-based revision of the original Compute Module, with a power quad-core processor. The Lite version does not come with the 4GB of on-module eMMC flash found on the standard Compute Module 3. It also does not contain the WiFi/Bluetooth chipset that comes with the Pi 3.
The Compute Module contains the guts of a Raspberry Pi 3 (the BCM2837 processor and 1GB RAM). The Pi 3 has a processor speed of 1.2GHz and runs at roughly 10 times the speed of the Pi 1 due to its quad-core CPU. This is all integrated onto a small 67.6mm x 31mm board which fits into a standard DDR2 SODIMM connector (the same type of connector as used for laptop memory). The processor interfaces are available to the user via the connector pins. You get the full flexibility of the BCM2837 SoC (which means that many more GPIOs and interfaces are available as compared to the Raspberry Pi), and designing the Module into a custom system should be relatively straightforward as the Pi Foundation has put all the tricky bits onto the Module itself.
If you need to cram a Pi 3 into a very small space or as the engine of your existing design, the Compute Module can go where no other Pi can!