C.C.D. is simply an abbreviated way of saying “Confraternity of Christian Doctrine”. It basically means religious education for children. Catholic kids who attend a Catholic school regularly already get religious instruction as part of the regular program. For Catholic kids who attend a public school, they get their religious instruction during CCD classes.
For those unfamiliar with the program, you can think of CCD as sort of the Catholic equivalent to children's Sunday school in Protestant churches. Like Sunday school, CCD provides children with a Bible (they are given a copy of the New American Bible translation, which is the translation most commonly used in U.S. Catholic churches.) The most obvious difference between the two is that CCD is usually not held on Sundays, but rather on weekday evenings. It also differs from most protestant Sunday school programs because Catholic kids not only get educated about Christianity, the Bible and Jesus, but they are also trained to receive sacraments such as Confession, Holy Communion, and Confirmation. Enrollment in the CCD program also does not fulfill the Sunday obligation for Mass attendance, so students are still expected to attend church on Sundays and their parents are encouraged to continue their Catholic instruction at home. CCD programs also occasionally have a Mass or two during the school year, but not weekly.
While Protestant kids focus on memorizing the Bible, Catholic kids get catechism. In other words, from preschool through the eighth grade, and from September to May, children attend class once a week to learn the Catholic faith. In past decades, many CCD classes were taught by clergy and Catholic religious leaders like nuns. Today, many CCD instructors are lay Catholics. Some famous celebrities who have served as CCD catechists include astronaut Ron Garan and comedian Stephen Colbert.
The Archdiocese of Chicago has put a renewed focus on recent years to get CCD catechists certified. However, any lay Catholic can volunteer to serve as a CCD teacher and many parishes are in need of more volunteers to serve as teachers. CCD teachers must must be confirmed Catholics, high school graduates, at least 18 years of age, and have no canonical impediments to the sacraments. CCD aides may be under the age of 18 and currently in high school. There are also other ways to volunteer to help a parish CCD program, including as office staff, coordinating special projects, retreats, and occasional help, as well as prayer partners and substitute catechists. Most parish CCD programs charge a slight fee for the classes, which covers the cost of texts, supplementary materials, art supplies, audio-visual and library supplies and administrative expenses. This fee is paid when the children are registered.
In conclusion, if you have grade school aged Catholic kids and they're not attending a Catholic school, you really need to enroll them in a CCD program if you want them to be raised in their faith and become practicing adult Catholics. Check with your local parish about their CCD program!