A aftermarket burner is one that is not a factory supplied burner like H-P. compaq, Dell, etc. These burners you buy and install yourself. If you take a look at the front of a cd drive(called a faceplate), generally it consists of a tray that pops out so you can put a cd in it, a button to open the tray, a headphones jack where you can plug in your headphones, and not much else. H-P made special covers for the drives to match the styling of the whole computer case. You may not be able to use this special h-p front panel(faceplate) on a cd burner you buy and install in the computer.
Installing the drive is easy. Heres a illustrated guide.http://www.fonerbooks.com/r_cd.htm
After you install the drive and turn the computer back on you will peobably be prompted by windows with a "Found New Hardware" popup message. Usually it will tell you what the drive is. If the drive is too new for windows to see it, it will ask you for drivers for the drive, and you should have a drivers CD(dvd) to install the drivers with the cdburner. Not all have drivers CD's with them so you may have to go to the cdburner makers website and download drivers to install. These are usually exe files that you just double click and they install themselves. You may not have to install any drivers at all if the computer sees the drive and identifys it correctly. Windows will tell you so pay attention to the message prompts you see.
Lite-on is a company that makes CDrom, CD Burner, DVD rom, and DVD Burner drives. They are known for good quality while being cheaper than most of the cddrive makers. Their drives are also perhaps 1/4 to 1/2 inch shorter than most cdrom drives. This shorter length can make the difference to where the drive fits in the case properly without sticking out of the front of the case and looking funny. I have a athlon 2600 computer that has a aluminum case that the standard cdrom drives hit on the edge of the motherboard causing the drive to stick out of the case 1/4 inch. By using a lite-on cd burner/dvd combo drive the drive faceplate is flush with the case.
The difference between a internal and external is that the external drives usually have a case of their own to protect the drive, and they usuall plug into a USB port on the computer. I prefer to use internal drives as i have enough spagetti wiring all over the place already. The ones i'm talking about here are internel drives. I do not like to use of external drives except for backing up customer data they want to save when they are upgrading or fixing a computer. The external drives are a bit slower than internal drives.
OEM just means the hardware usually does not come with drivers cd's since it is a manufactured computer. OEM stands for Original Equipment Manufacturer. H-P, Compaq, Dell, Gateway, etc are OEM companys.