At some point, Atari split into two companies. Atari Games was essentially Atari's arcade division and they were able to use the Atari name on arcade releases but not on home ports. Atari Corporation was responsible for home consoles and computers and had the rights to the Atari name on these. When Atari Games decided to develop games for the home market, Tengen was created as a subsidiary since they could not use the Atari name for these home ports.
This ad shows a few of Tengen's products for the Sega Genesis. There isn't a whole lot to be said for Ms. Pac-Man and Pac-Mania. These were arcade games originally and the Genesis was powerful enough to produce arcade quality conversions. Hard Drivin' isn't quite so clear cut. It isn't a terrible conversion and it got pretty good reviews at the time but it isn't a game that has really held up well over time.
Tengen released a variety games and a large number of systems over the seven or so years of their existance, including both licensed and unlicensed games for the NES. These three games have seen numerous releases on various systems and as part of various collections.
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