CD Duplication (copying of CD's) has become very popular because it is very easy to make and inexpensive. It may also be a subject of controversy with all legal issues relating to copyright, so you should know. Laws and regulations are complex and vary from one country to another state - so you'll need to check yourself to ensure that no illegally duplicating CDs. If you use CD duplication for personal use and copy files from your computer or CD you own then you should not worry too much about it. If you make multiple copies of a commercial CD for distribution to friends and, most important sale for the general public, then you really need to make sure you have the legal rights to do so.
The fundamentals of CD duplication is simply copy a CD to another blank CD. The CD copy can be anything you bought at your local music store (beware of copyright) or a CD that you borrowed from a friend. The CD may contain all kinds of things outside of music - photos, movies, data files, MP3 files, etc, etc CD duplication at home using a simple software installed on your computer is very easy to do if your computer has a CD-RW (if not, then you can buy an external CD-RW drive and connect it to your computer).
You need special software to work with the duplication of CDs on your computer, but if your computer has a CD-RW then you will probably find that your computer has software installed to work with CD's (if your buying a CD RW External then it also comes with the software can be downloaded to your computer) - check this out before buying one of many software products available.
Most programs are "generalists", allowing you to duplicate many file types. However, with this type of software is usually a level of commitment in terms of editing features and quality of material doubled. If you're only interested in saying that the copying of music files, then you might want to buy software that will give you plenty of useful editing functions and provide a better quality of duplicate content.
The same applies to blank CDs - some are more appropriate for data files and others for music, etc - to choose the software and blank CD carefully and see if you are unsure. Also, if you want to be able to modify or change the content of their double CD at a later time, so be sure to choose a CD-RW drive not CD-R.
Beyond basic home copying of CDs, if you're a serious amateur or CD duplication for commercial reasons, then you may consider some of the equipment and more specialized software available.