Hibernate and sleep functionalities in computer and better way to save electricity

What are the functionalities of sleep and hibernate in computers? Which one is better if I wanted to only use a little electricity on my computer while I am not working on it. When in Hibernate mode, should the computer turn off and that I need to press the Power button again when I decide to use it again? Should it take a few seconds to turn on the computer from hibernate mode? Please help.
