Chemical elements are the basic building blocks of our reality, and constitute everything from stars and planets to the air you're breathing and even possibly Mountain Dew (unconfirmed).
All elements are made up of protons, neutrons (except for Hydrogen) and electrons. Protons (positively charged) and neutrons (neutral charge) make up the nucleus of an atom, which electrons (negatively charged) orbit the nucleus in shells. Isotopes are elements with a different neutron count than normal, and may be stable or unstable (radioactive). You are now prepared to teach chemistry on the 9th grade level (you're welcome).
Most elements readily form molecular bonds with each other, such as Hydrogen and Oxygen into water. Some bonds are stronger than others depending on the way they've bonded. Elements also bind with themselves into molecules such as Hydrogen gas. Everything around you is made up of molecules, including your computer, your mug and the coffee in it, and your dog's butt.