It has two main features.
1) Burning CDs/DVDs. This is a pretty standard action that a lot of programs do, and I don’t think needs much explanation.
2) Disk emulation. Alcohol 120% can create an image of a CD/DVD (A virtual copy on your harddrive). You can then run the image on a virtual drive, just like you would run that DVD in a DVD player.
Why would you do that? Well, you could lose the disk and wouldn’t have to worry about it. In some cases, Harddrive read-times are faster than those of CDs/DVDs, and you don’t have to deal with the noise, you can distribute those image files across the internet (Note: This is not legal to do, unless you are the creator of the contents that were on the DVDs), and there are some security benefits.