Elements: the ancient Greeks thought that there were only four elements: earth, air, fire and water. They thought that every substance was made of some combination of those four.
However, thousands of years of chemistry experiments showed that those four "elements" weren't as simple as the Greeks had thought; water, air, and earth, for instance, could all be broken down into simpler "ingredients."
However, as chemists worked, they built up a list of materials that couldn't be separated into more basic ingredients. They called these materials the chemical elements. It turns out that there are 92 naturally-occuring elements that are the building blocks of all matter. There are 92 naturally-occuring kinds of atoms, and all matter is made of some combination of those kinds. (There are also elements beyond 92, but those are artificial - made if physics and chemistry labs.)
You can think of elements as tiny "Legos;" there are 92 kinds, all different from one another. Everything made of matter - all solids, liquids, and gases - are made of these elements, in different combinations.
A compound is a material whose molecules are made of more than one kind of element. A compound is still a pure substance - it's NOT a mixture. Every molecule of a compound is like every other molecule. But each molecules is made of more than one kind of element.
H2O (water), for example, is a compound. Each molecule of water is made of two atoms of hydrogen and one of helium.
In project H, you'll be investigating other compounds.
Another write-up on elements can be found at here, at Chem4Kids.
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