The Raspberry Pi foundation has come out with three models to date. All are different in various ways.
The Model B+ was launched in July, 2014 and is the latest addition to the Raspberry Pi line. 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.