The Sega Genesis was released September 1st, 1989 in the US. In other countries, the Genesis was called the MegaDrive.
Sega was not going to make the same mistake they did with the master system, so they released the Genesis before Nintendo could release their 16-bit competitor.  And, for a while, the genesis had no real competition.   However, because of Nintendo’s monopoly over 3rd party developers, the genesis did not flourish like it should have.

In November 1992, Sega released the Sega CD, an add on unit that allowed you to play games that came on CDs.  It also improved sound, and sprite features, but it did NOT improve the color, an area which needed a lot of improvement.  With the CD, you could hold hundreds of times more information compared to the cart.

In November 1994, Sega released the 32X add on unit.  When plugged into the cartridge port of the machine, provides extra processing power, graphical enhancement and other improvements.  The 32X greatly increased the number of colors.  It could also be used in conjunction with the Sega CD.

Eventually, similar to what Sega did with the master system, smaller, more compact versions of the Genesis were released.  Including the Genesis II, the amazingly tiny Genesis III, and the hand held Nomad, similar to the game gear.

Many accessories were released for the genesis, including the Power Base Converter.   This little device would allow you to play almost all master system carts and cards on the genesis.  This expanded the number of games you could play by a couple of hundred.