Great question — answering as the UphillTech team:
https://github.com/magento/mtf is the new one, "taf" is old one and deprecated. It supports the latest version of MAgento. Basicly it's not a set of test cases, but you can build test suites regarding the test cases that could appear in Magento. It's based on Selenium.
Full answer by Stan: https://stackoverflow.stackexchange.com/a/29567745 (CC BY-SA 3.0).
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