The MacBook Pro 13" with Retina display was introduced in October 2012, following the introduction of the 15" model the previous June. Its defining feature over the non-Retina 'unibody' model was its very high resolution 2560x1600 display. It also added a second Thunderbolt port, HDMI and USB 3.0. However it lost its DVD drive, along with ethernet and FireWire 800 ports (Thunderbolt adaptors for ethernet and FireWire 800 were sold separately).
It is much less upgradeable than previous models, with
RAM being soldered onto the board and non-replaceable, the
battery glued in place, and the SSD drive, while being upgradeable,
is of a non-standard type. The case is put together with pentalobe screws.
The first, late 2012, revision was released with a 2.5GHz
Intel Core i5 CPU, with a 2.9GHz Core i7 build to order option.
All models were sold with 8GB of 1600MHz DDR3 memory,
and there were 128GB, 256GB, 512GB and 768GB SSD options.
Apple specs here.
The line was refreshed in February 2013 - while still offering
the 2.5GHz Core i5 (with 2.9GHz Core i7 BTO option) a 2.6GHz
Core i5 (with 3.0GHz Core i7 option) was added. Apple specs here.
New models were released in July 2014, with very little differences to the previous models beyond slightly increased CPU speeds - 2.6GHz and 2.8GHz i5 and 3.0GHz i7. Apple specs here.
The March 2015 revision was outwardly very similar, but did see some significant upgrades to CPU and GPU architecture, and a faster SSD and RAM (the latter still unfortunately soldered into place to prevent after-purchase upgrades however). This revision was sold with 2.7GHz and 2.9GHz Core i5 models, with a 3.1GHz Core i7 Build-To-Order option. Apple specs here.
In Late 2016 a new revision came out, both with and without the new Touch Bar. All models have soldered in RAM, and the Touch Bar models also have a soldered SSD, making them completely non-upgradeable. The models without the Touch Bar do have removable SSDs, but of a non-standard type which are incompatible with the third party ones made for earlier MacBook Pro Retinas.