The Sega CD is an add-on for the Genesis that allowed you to play games stored on a
proprietary CD format. Unfortunately, it didn't offer any resolution or palette
enhancements to the Genesis. It did have an additional 68000 processor (remember, the Genesis had another one), which
allowed for the creation of computation-intensive games like Silpheed. It also had CD-quality
sound and lots of storage space (compared to cartridges), which further enhanced the gaming experience.
The Sega CD ended up having more than 100 games, most of them garbage.
I am still happy I got my Sega CD, though.
Lunar, Lunar 2, Sonic CD, Silpheed, and Eternal Champions made it a worthy purchase.
These are some of my favorite games for the Sega CD, listed by category: