Since its release, Raspberry Pi has come in various iterations, featuring both small and large updates and improvements to the original Raspberry Pi Model B unit. Although it can be confusing at first, there are three basic types of Raspberry Pi available (and one special model).

The main flagship model is called Model B. This has all the connections and features, as well as the maximum RAM and the latest processor. Over the years, there have been several versions, most notably Model B (which had 256 MB and then 512 MB RAM) and then Model B+ (which increased the 26-pin GPIO to 40 pins, switched to using a microSD card slot, and had four USB ports instead of two). These original models all used the Broadcom BCM2835 system on chip (SOC), consisting of a single core 700 MHz ARM11 and VideoCore IV graphical processing unit (GPU).

The release of Raspberry Pi 2 Model B (also referred to as 2B) in 2015 introduced a new Broadcom BCM2836 SOC, providing a quad-core 32-bit ARM Cortex A7 1.2 GHz processor and GPU, with 1 GB of RAM. The improved SOC added support for Ubuntu and Windows 10 IoT. Finally, we had the latest Raspberry Pi 3 Model B, using another new Broadcom BCM2837 SOC, which provides a quad-core 64-bit ARM Cortex-A53 and GPU, alongside on-board Wi-Fi and Bluetooth.

Model A has always been targeted as a cut-down version. While having the same SOC as Model B, there are limited connections consisting of a single USB port and no wired network (LAN). Model A+ again added more GPIO pins and a microSD slot. However, the RAM was later upgraded to 512 MB of RAM and again there was only a single USB port/no LAN. The Broadcom BCM2835 SOC on Model A has not been updated so far (so is still a single core ARM11); however, a Model 3A (most likely using the BCM2837).

The Pi Zero is an ultra-compact version of Raspberry Pi intended for embedded applications where cost and space are a premium. It has the same 40-pin GPIO and microSD card slot as the other models, but lacks the on-board display (CSI and DSI) connection. It does still have HDMI (via a mini-HDMI) and a single micro USB on-the-go (OTG) connection. Although not present in the first revision of the Pi Zero, the most recent model also includes a CSI connection for the on-board camera.

The special model is known as the compute module. This takes the form of a 200-pin SODIMM card. It is intended for industrial use or within commercial products, where all the external interfaces would be provided by a host/motherboard, into which the module would be inserted. Example products include the Slice Media Player (http://fiveninjas.com) and the OTTO camera. The current module uses the BCM2835, although an updated compute module (CM3).

The Raspberry Pi Wikipedia page provides a full list of the all different variants and their specifications:
https://en.wikipedia.org/wiki/Raspberry_Pi#Specifications

Also, the Raspberry Pi product page gives you the details about the models available and the accessories' specifications:
https://www.raspberrypi.org/products/