The Raspberry Pi foundation has come out with three models to date. All are different in various ways.
The Model A+ was launched in November, 2014 and is the latest addition to the Raspberry Pi line. The A+ has a smaller footprint than the prior Pis, cutting almost an inch off the length of the board. It comes with 256Mb RAM, 1 USB port and no ethernet jack, similar to the original Model A. The most groundbreaking aspect of the board though, was the price. It was intriduced at a mere $20. That, combined with the smaller size made it the least expensive, smallest single board computer in the world at the time of release. At least for the time being, the Model A+ is only being produced in limited quantities.
The Model B+ was launched in July, 2014. It's different than the B and A models in that it has four USB ports, a longer GPIO connector and usees a micro SD card. For those reasons, many legacy accessories may not fit, or require adapters.
The Model B was launched in February 2012 and is the most popular version of the Raspberry Pi to date, with millions of boards sold worldwide. The model B featured an ethernet jack, but is otherwise dimensionally the same as the Model A. Many accessories which are compatible with the Model B also work with the Model A.
The Model A was launched after the Model B as a lower cost, lower power alternative. As stated above, the Model A was missing the ethernet jack of the Model B. It also only had one USB port, as opposed to two, like the Model B. Many accessories which are compatible with the Model B also work with the Model A.