How CDs work
All three CD disc types (CD-ROM, CD-R, CD-RW) store information in grooves that are 1.6 microns wide (a micron is one one-thousandth of a millimetre; the average human hair is about 50 microns wide). Unlike magnetic storage media, such as hard disks, which store data as polarised particles on a magnetic surface, CD-Rs and CD-RWs store data as microscopic reflective and nonreflective spots along the grooves. A drive reads the disc by shining a laser onto its surface and noting how the light reflects (or doesn't) off these spots in the grooves.
Mass-produced audio CDs and CD-ROMs are stamped by small presses that create tiny bumps (called lands) and holes (known as pits) in the grooves. Pits reflect light differently than lands along a disc's aluminium or gold surface, and the laser tells the difference by measuring the brightness of the reflection. CD-Rs and CD-RWs contain light-sensitive dyes or chemicals embedded beneath layers of protective plastic. When the high-intensity recording laser hits these light-sensitive materials, they become reflective (or not).
Drive speeds: Write/Rewrite/Read
What exactly does "24x10x40" mean? These three numbers refer to the recording, rewriting and read speeds respectively. So, if a device has the specifications "24x10x40", it would record a write-once CD at 24-speed, record to rewritable CDs at 10-speed, and read CDs at 40-speed. If this doesn't mean much to you, it may help to think of these figures in terms of playing an audio CD. Playing a CD is the same thing as "reading it at 1-speed". For a standard CD that is filled to capacity, this will take 74 minutes. Therefore, reading a CD at 40-speed will take one-fortieth of that time, which is roughly two minutes.
These speeds are usually calculated as an average, so be prepared to accept a little lower performance than claimed in the specifications, particularly when it comes to read speeds. At the time of writing, 40x write burners are the fastest available, with 12x to 24x recording being pretty standard.
Media types: CD/CDR/CDRW/DVD/DVDR
These acronyms are often used loosely, and sometimes interchangeably, for the drive and the media used by the drive. A CDR or CD-R refers to a recordable CD disk, or a CD drive that can write to blank CD disk. A CDRW or CD-RW refers to a rewritable CD disk, or a drive that supports recording to rewritable disks. All CDRW drives will write to CDR media. Not all CDR drives will write to CDRW media, although this is not so much a problem as it is virtually impossible to buy a plain CDR-only drive these days! Generally speaking, the term "CD" is used as a blanket term for both CDR and CDRW disks. Some CD media can store 700MB of data or 80 minutes of audio, although 650MB and 74 minutes is the standard.
A DVD (or DVD-ROM) drive will read all CD, CDR and CDRW disks as well as DVD disks such as those used for DVD video. While DVD almost always refers to video disks, DVD media can be used for other data, as if they were high capacity CDs. These writable DVD disks are called DVD-R. A DVDR drive is a DVD writer which will read and write to CD media as well as DVDR disks. A DVDRW is the rewritable version of DVDR. DVDs can contain up to 4.7GB of data, which is almost seven times the capacity of a CD.
Internal vs External
In the majority of cases, you'd be looking at purchasing an internal drive. These will always be cheaper and do not require any extra desk space. Bear in mind that they will require installation, although this is a simple procedure, even if you have never delved inside your PC's casing before! Buying an internal drive also gives you the option to convert it to an external drive should the need arise- a luxury you don't get with a specialised external drive.
All this aside, there are quite a few advantages to external drives, so the extra cost might be worth it to you. Firstly, an external drive is portable. It also enables you to write CDs from any PC, so if you are printing or working on one, you can burn on another. Additionally, a removable drive will usually connect via USB or FireWire port, so it won't use up an IDE connection in your PC. This means that you won't be restricted to the four drive connections that are standard on most motherboards.
On the downside, you need extra desk space for the box, and all your PCs will need compatible connectors- be it USB, USB2, FireWire etc. If you're not sure about getting an external drive, there is always the option to buy an external case sometime down the track. This will let you use an internal writer as an external writer- and possibly free up an IDE connector, as many external units available will convert the IDE interface on the drive to FireWire or USB.
There are essentially five types of connection options for CD writers. Internal drives can be IDE, which is the same interface as regular hard disk drives, or SCSI. Unless you require SCSI for connectivity reasons, such as use with musical instruments or video equipment, it is not usually worth the extra expense.
For external burners, you have a few more options. Firstly, there is USB. Most PCs built in the last few years will be equipped with USB connectors so this is a good option for compatibility. Bear in mind, though, that the newer USB2 format is much faster although less widely implemented except in very new PCs. Be careful when buying a USB2 device that your PC will be able to use it!
Next up, there is FireWire, also known as IEEE 1394. This is a high-speed connector, much like USB2, found commonly in multimedia notebooks but less so in desktop PCs. If you use FireWire for editing video from a DV camera, then this might be a good option for you. Otherwise, you'll need to upgrade your PC with a FireWire card, which not only costs around $100, it will also use up a PCI slot in your motherboard.
Last off the ranks is the PCMCIA interface. This is the "PC Card" format used in almost every notebook ever built. This is a good option for non-FireWire equipped notebooks, but not really of great use to the desktop user. That is, of course, unless you happen to have a PCMCIA adapter for your PC.