The Raspberry Pi Model A is a cut down version of the Model B, featuring a smaller form factor and only one USB port. The Model A’s do not include ethernet, and are designed to be lower cost, lower power consumption alternative to the Model B.
The Raspberry Pi Model B is the flagship Raspberry Pi Model, with the fastest CPU and most connectivity features. The latest revision (Raspberry Pi 3 – Model B) features a 1.2GHz Quad Core processor, and on board WiFi & Bluetooth. The Model B is the largest Pi available with the approximate size of a credit card.
The Compute Module features a 200 Pin DDR2-SODIMM layout, and is designed for engineers who need Raspberry Pi functionality but need a smaller form factor for embedded designs.
The Raspberry Pi Zero is an ultra low cost and ultra small variant of the Raspberry Pi. It comes with an unpopulated GPIO header, and only a single micro USB port for connectivity. The Raspberry Pi Zero W (Wireless) includes on board WiFi and Bluetooth.