The Raspberry Pi 2 was introduced in 2015. It improved upon the Raspberry Pi 1 by using a more powerful processor and additional RAM, while holding the price the same. Importantly, the Raspberry Pi 2 also increased the number of GPIO ports from 17 to 26. Unfortunately, in doing so the GPIO pin configuration was changed, which means that projects created for the Raspberry Pi 1 are not compatible with projects created for the Raspberry Pi 2.
In 2016, the Raspberry Pi 3 was introduced. It improved upon the Raspberry Pi 2 by adding a more powerful processor, faster RAM, and built-in wireless networking. (Wireless networking with a wireless Raspberry Pi 2 was usually done using a wireless adapter inserted into one of the USB ports.)
The price of all three versions of the Raspberry Pi has been $35.
The first-generation Raspberry Pi 1 was available in two models: Model A and Model B. The Model A version was a slimmed-down version of the Model B, with less memory and fewer I/O options but consuming less power. In 2014, these two models were replaced with the Model A+ and Model B+, which added additional capabilities.
One of the main differences between the Model A+ and Model B+ is that the Model A+ does not have built-in networking. Because connecting to a network greatly simplifies the task of configuring your Raspberry Pi, avoid the Model A+ until you've gained some experience working with the more powerful Raspberry Pi models.
Note that versions 2 and 3 of the Raspberry Pi are not available in A or B models.The original Raspberry Pi 1 models (both A and B) had fewer GPIO pins than the newer versions. The original versions provided a 26-pin header block that provided access to 17 GPIO ports. On the newer versions, including the Raspberry Pi 2 and 3 models, a 40-pin header provides access to 26 GPIO ports.
If you don't yet own a Raspberry Pi, and you're planning on working on your own electronic gadgets, you should purchase a Raspberry Pi 3 as part of a kit that includes the materials you'll need to get started. These simple kits include a power supply to power your Raspberry Pi, a microSD card already loaded with the operating system software, and a small book to get you started. Some kits include additional goodies, such as an HDMI cable to connect your Raspberry Pi to a monitor and perhaps additional components such as LEDs, resistors, jumper cables, and a breadboard.