Amino acids are fundamental building blocks for proteins. Proteins are made of long chains of amino acids. Sometimes these chains can be thousands of amino acids long. They twist and turn, with different parts of the amino acids interacting with each other, ultimately forming the complex structures of proteins. Proteins, in turn, are key components of many cellular structures - you can't live without proteins.
There are 20 amino acids found in human proteins. They are all chemical compounds, containing several different types of atoms. All amino acids have an amine group, made of nitrogen and hydrogen (NH2). They also have a carboxyl group (acid - COOH). The rest of the molecules contain various amounts of carbon, hydrogen, and nitrogen. Although they all share the same amine and carboxyl groups, it's the functional "other parts" that make one amino acid different from another.
Amino acids are used for other things in the body aside from just being the basic units of proteins. They are essential for forming many chemicals, such as hemoglobin, the chemical in your blood that carries oxygen. They play important roles in nutrition and metabolism as well. The study of amino acids is quite complex, there is no way to even outline all of the functions they serve in the space available here.
Amino acids are often broken in to two categories, essential and non-essential. Essential amino acids are those which your body is unable to synthesize itself. This means that you much inject these amino acids in your diet. Non-essential amino acids are those which your body can make. It is not essential that you eat those for your body to have enough of them.
The essential amino acids are as follows: lysine, leucine, isoleucine, methionine, threonine, phenylananine, valine, and tryptophan.
The non-essential amino acids are as follows: alanine, arginine, asparagine, aspartate, cysteine, glutamate, glutamine, glycine, histadine, proline, serine, and tyrosine.
As is that case with many things in biology, the essential, non-essential issue is actually slightly more complicated than this. Children are unable to make several of the non-essential amino acids in proper quantities, making them "semi-essential" until the metabolic pathways are fully developed.
When amino acids form chains to make proteins, they form a very specific type of chemical bond. This bond is called a "peptide bond". This bond has unique properties which allow the forming protein chain to take on it's proper shape. Ultimately, it is the shape of a protein that determines its function. Therefore, the sequence of amino acids is what ends up determining the function of a protein. There is now an entire branch of science, known as proteinomincs, which deals with predicting the function of a protein based on only knowing the basic amino acid sequence. Needless to say, this is not an easy branch of science.
There is almost literally an infinite amount of knowledge that can be studied on the issue of amino acids. They are vital to our lives in many, many ways.