You are awake when you are not asleep. When you are awake, usually your eyes are open and you can move around.
When you are asleep, sometimes you dream. Dreams are a little like a movie that plays in your head while you sleep. Sometimes dreams are very ordinary, sometimes they are exciting, and sometimes they are scary. Scary dreams are called nightmares. Often things happen in dreams that aren't possible in real life.
A nap, sometimes called a kip, snooze or siesta, is a short sleep, usually in the middle of the day after lunch.
A nightmare is a bad dream. Nightmares can be extremely frightening, and make you wake up scared. You might also use nightmare to describe a particularly bad event, for example "we had a nightmare vacation".
You take a rest when you stop working and be still for a time. Once you have had a good rest, such as the morning after a good sleep, you are well rested.
A siesta is a short sleep taken in the middle of the day, particularly by people in Spain or Latin America. A similar word to siesta is nap or snooze.
Sleep is a time of rest, usually for about eight hours at night time. People usually sleep in beds. When you are asleep, you usually have dreams, but not everyone can remember their dreams the next morning.