Atoms, molecules, compounds -- chemistry can be confusing at first. However, it becomes easier once you learn the concepts behind the terminology. One of the most important ideas to understand is that of a chemical compound. Originally, the word "compound" meant "to put together" or "to join." Put simply, a compound is a substance in which two or more elements are chemically bonded.
At Least Two Different Elements
A compound contains at least two different kinds of atoms. Another way to say this is that a compound is a substance made up at least two different elements. Oxygen, O2, is an element because it has two of the same kinds of atom. Water, H2O, is a compound because it has two different kinds of atoms -- oxygen and hydrogen.
Defined Ratios of Atoms
The atoms in a compound have a fixed ratio. This means that every molecule in a compound will always be exactly the same. A water molecule will always have one atom of hydrogen and two atoms of oxygen.
Compounds can be chemically broken down into simpler substances. These substances will always be elements or other, simpler compounds. For example, water can be separated into two elements -- hydrogen and oxygen. Notice that this is another way of thinking about the fact that compounds are made up of at least two different elements.
Compounds cannot be physically separated. Another way to say this is that compounds are joined by chemical bonds. For example, sugar water is a mixture; you could separate the sugar from the water by allowing the liquid to settle or by putting it into a centrifuge. Water is a compound. You cannot separate the hydrogen from the oxygen without a chemical process to break the bond between the atoms.
Compounds have a particular set of characteristics. Because the molecules of a compound are always exactly the same, the properties of the compound will always be exactly the same. For example, water will always freeze at zero degrees Celsius and boil at 100 degrees Celsius.