Proteins

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The proteins are a huge family that make up about half the the human body's dry weight. They are found everywhere in all living organisms. They can function as a building material, in teeth and bones and muscles, and they can serve as enzymes, hormones, and neurotransmitters. It's functions is the most diverse of any family. The word protein comes from the Greek proteios, or "of first rank."
Proteins consist of macromolecules called polypeptides, made from monomers called amino acids. Most proteins also include traces of other organic molecules or metal ions, which give it its characteristic biological function.

Amino Acids

The monomer units for polypeptides are a group of about 20 alpha-amino acids, all of which share the following strucutral features. 

or more simply:
NH2-CH-COOH     +NH3-CH-CO2-
    |       ==>      |
    G                G
                (dipolar ionic form)
Amino acids are usually found in their dipolar ionic form because of internal self-neutralization. A proton (hydrogen nucleus) is transfered from the proton-donating carboxyl group to the proton accepting amino group.
The G stands for a side chain. There are about 20 side chains which give the 20 different amino acids. The simplest one is just an H, which makes glycine (Gly). 
+NH3CH2CO2-
glycine

Polypeptides

When amino acids are joined together, they make polypeptides. The carboxyl group of one amino acid becomes joined to the amino group of another by a peptide bond (amide bond). For example, this is the linking of glycine and alanine: 
  H                      H                       H
   \ +                    \ +                     \ +
H - N - CH2 - COO-  +  H - N - CH - COO-  ==>  H - N - CH2 - CO - NH - CH - COO-  +  H2O
   /                      /    |                  /                    |
  H                      H     CH3               H                     CH3
   glycine (Gyl)    +     alanine (Ala)   ==>     glycylalanine (Gly-Ala)         +  water
Glycylalanine is a dipeptide because it has two amino acids linked by a peptide bond (CO - NH). A different dipeptide could be made using the same two amino acids, but putting them in a different orientation. This would make alanylglycine (Ala-Gly). The three letter abbreviations can be used to represent the structural formulas of the amino acids.
Also, because the molecules produced are still dipolar ions, more amino acids can be put together. Proteins have hundreds, thousands, or sometimes even millions of these amino acids.

Proteins

Some proteins consist of only single polypeptides. But most involve two or more aggregated polypeptides, sometimes with other small organic molecules or metal ions.
Hemoglobin, the substance in your blood that carries the oxygen, has four polypeptides (two similar pairs), and one molecule of heme, which colors the blood red. Heme holds an Fe2+ ion. If one piece of it is altered in any way, the substance will cease to function, or will function improperly.
Shape is also important. Proteins are coiled and twisted, giving it a unique shape. The shape it critical in the ability of the protein. The shape depends on the sequence of the amino acids, which can be hydrophilic or hydrophobic. Those that are hydrophilic want contact with water and will be twisted to maximize contact. Those that are hydrophobic are twisted in such a way to minimize contact. Changing one amino acid in the polypeptide sequence can destroy this shape and make the protein function improperly if not at all.
The shape can also change when protons are donated to accepting sites or released from donating sites. Thus, any change in the pH can change the shape drastically. Because of this, living systems must have tight control over the pH of its fluids by means of buffers.

Enzymes

A very important function that proteins can serve are as catalysts. Enzymes are organic catalysts made of proteins. They speed up reactions inside an organism. The molecule which an enzyme catalyzes is called a substrate. Enzymes can only act on the substrate that they were designed for. This is again because of protein shape. If the substrate molecule's shape matches the enzyme's active site, it undergoes the reaction specified. This is called the lock-and-key theory of enzyme action.
(Image of lock-and-key theory)
Enzymes can either break or put together substrates. And they can enhance the rate of reactions to over half-a-million molecules per second. Because of the lock-and-key theory, enzymes must retain their shape to keep their function. If the temperature is too high, the enzyme will change its shape, becoming almost like a random mass of coils, in a process called denaturation, and thus losing its function. Some dangerous poisons work by deactivating enzymes by changing their shapes.
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