Egyptian hieroglyphs on a temple wall
Hieroglyphs means "sacred drawings" in Greek, and that's pretty much what hieroglyphs are. The Egyptians used them to write with from the beginning of anybody being able to write, around 3000 BC, down to about 300 AD. Between about 3000 BC and 300 BC, the Egyptians used hieroglyphics for all different kinds of writing. But after the Greeks conquered Egypt under Alexander the Great, people began to use the Greek alphabet to write the Egyptian language. Then hieroglyphs were only used for religious things (ta hiera in Greek), things that were too holy for the ordinary Greek alphabet, which is why they are called "sacred-drawings." By 300 AD, as people converted to Christianity, there was no longer any religious use for hieroglyphs, and they went out of use altogether.
Hieroglyphs are basically drawings of familiar objects, simplified to make them easier to draw. At first people just drew a dog or a house or a sheep; for example some early writing is just a picture of a sheep with five lines by it to mean "five sheep." Then people began to combine pictures, so that a picture of a sheep means the sound "sh"and can be combined with a picture of an owl "hoot" to mean the word "shoot," for example (only in Egyptian of course, not in English really!).
To find out more about Egyptian hieroglyphs, check out these books from Amazon.com or in your local library:
Hieroglyphs : The Writing of Ancient Egypt, by Norma Jean Katan and Barbara Mintz (1981). An introduction for kids, with historical context.
Hieroglyphs, by Joyce Milton (2000). With stencils, so kids can write their own names and other things in hieroglyphs.
The Hieroglyphs of Ancient Egypt, by Aidan Dodson (2001). Well illustrated, and the author is an expert.
The Mystery of the Hieroglyphs: The Story of the Rosetta Stone and the Race to Decipher Egyptian Hieroglyphs, by Carol Donoughue (1999). For kids, about how modern people figured out how to read ancient Egyptian hieroglyphics. A Parents' Choice book.