bear in mind the difference between a dvd "burner" under $100 single or dual they are used for burning data, meaning to use it your movie or TV program must be turned into bits and bytes, a digital file. This requires software and a 2 step process.
a dvd "recorder" more on the $350 - $1000 range depending on sophistication, works exactly like a VHS recorder. It records direct in real time, a one step process. The difference in quality is related, as Fez mentioned to the amount you try to fit on one disc. The XP setting on my Panasonic records only 1 hour but it is great quality. As you move down to the 4th setting, getting 6hrs the image quality quickly deteriorates.
The prices on these real time recorders has dropped by about 40% from last year. Next year the lesser quality ones should be had for about 150 -199. Don't confuse a "burner" with a "recorder", BIG difference.