Hydrogen peroxide is two atoms of oxygen, each attached to a hydrogen atom. You can think of it as a water molecule with an extra oxygen in the middle.
The oxygen we breath has two oxygen atoms in it – we call it molecular oxygen. In molecular oxygen, the two atoms share two electrons, in what we call a double bond.
When two oxygens are connected with a single bond, we call it a peroxide. Hydrogen peroxide is the simplest example of a peroxide molecule.
Pure hydrogen peroxide is a liquid a little heavier and thicker (more viscous) than water. It is a strong oxidizer, and can start fires if it contacts flammable materials like wood or paper.
In the home, dilute solutions of hydrogen peroxide are used to disinfect and bleach. 3% hydrogen peroxide is used to clean wounds and as a mouth rinse to kill bacteria. Stronger solutions are used to bleach hair.
Cells produce hydrogen peroxide as a byproduct of metabolism. But hydrogen peroxide is also used by the immune system as a signal that attracts white blood cells to an infection.
A 3% solution of hydrogen peroxide will produce ten times its volume in oxygen if a catalyst (such as blood or dry yeast) is added. Most common metals will also act as a catalyst for the dissociation of hydrogen peroxide into oxygen and water. Adding a catalyst to hydrogen peroxide is thus an easy way to generate oxygen.