http://www.blurtit.com/q738440.html "-ful" is a suffix. It is the opposite of "-less". If you need more help, you can google suffix -ful.

It really depends on the word. Restful more or less simply means "full of rest". Usually -ful is added to make something an adjective, but you can't do this with every word. "I hope you get a lot of rest!" (noun) "I hope you have a restful vacation!" (adjective)