The Raspberry Pi 3 is exactly the same physical dimensions as the Raspberry Pi 2 and Model B+. It boasts a quad-core 64-bit 1.2GHz Processor, and has 1GB Ram. It also includes on-board 802.11 b/g/n WiFi and Bluetooth 4.1.
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The Raspberry Pi 2 Model B has a Quad-core 900MHz Processor, 1GB RAM, 4x USB Ports, 40x GPIO Pins, microSD connector and full 1080p HDMI video support. It is the first edition of the Raspberry Pi that is built to support Windows 10 IoT. (This item posts for free)
The Raspberry Pi 3 Model B+ (Released March, 2018) has a quad-core 64-bit 1.4GHz Processor, 1GB Ram and onboard Dual-band 802.11ac wireless LAN and Bluetooth 4.2 (including BLE).
This edition allows for PXE network and USB mass-storage booting and has a Gigabit Ethernet Network Interface.
The Original Raspberry Pi Model B
The Raspberry Pi 1 Model B+ has a 700MHz Processor, 512MB RAM, 4x USB Ports, 26x GPIO Pins, SD card connector and full 1080p HDMI video support. Price includes a matching Multicomp Raspberry Pi case.
The Raspberry Pi Zero WH is the smallest Raspberry Pi. It has onboard WiFi and Bluetooth. It has a micro SD card slot, a mini HDMI port, two micro USB ports (one for power, one for USB), and 512MB of RAM, and is powered by a single-core 1 GHz processor.