What is the difference between a single layer and a multi layer DVD?

If you copy a multi layer DVD to a single layer DVD, do you loose quality?What is the difference between a single layer and a multi layer DVD?
multip layer dvd is always DVD 9 (which is larger than 4.7GB ) while single layer DVD alwasy DVD5 (which is smaller than 4.7 GB); if you want to copy a DVD 9 to a DVD5, you need to compress the DVD, and doing this will lost quality more or less indeed, however, if the compression ratio is larger than 60%, the quality can meet my need.





I always use Ideal DVD Copy to backup my DVD collection, it support to copy DVD9 to DVD 9 and can also support to compress the DVD 9 to DVD5; further more, it can split a DVD9 to two DVD5, I like this copy mode, then you can use single layer disc instead of double layer disc and keep the original quality, you may free download from http://www.idealdvdcopy.com/ and have a try by yourself. There is a guides on that page show you the detailed information.