I would like to make a copy of some DVDs I own to let others borrow without risking loss of the originals. I have several questions:

1. Should I use DVD+ or DVD-? What is the difference and how do I tell which I need to use?
2. What is the significance of something like 16x, relative to DVDs I can purchase?
3. What can I do if the original is 4 hours and the capacity of the DVDs I buy is only 2 hours? Can I spread the original over 2 DVDs? How?

Look here: What is DVD?

and also under LISTS

1. Depends on what the source will record and the target player DVD-R best choice.
2. 16x is the max recording speed. It also depends on what burner is capable of.
3. I can't answer.

1. "What's the Difference between DVD+R, DVD-R, DVD-RW, DVD+RW, and DVD-RAM?" from the Ask Dave Taylor! Tech Support Blog

2. the speed written on the DVD cover tells you how fast stuff can be written to the dvd. If you buy a 8x dvd with a dvd-writer that can do 16x it means that the dvd writer can only write @ 8x. But if u buy a 16x dvd the writer will write @ 16x.
BUT if you get a 16x dvd and your writer can only write @ 8x then the writer will only write @ 8x.

3. yes you can spread the original over 2 dvds, but the process is different for different formats (movie, music etc). Try using Google to find specifc help.