As mentioned in the question, all the proteins in our cells (and there are many - diverse as they are numerous) arise essentially from four nucleotides - A, C, G, and T - in DNA. These stretches of DNA (known as genes) code for mRNA (the "m" standing for messenger, as these molecules carry a message from the nucleus to the cytoplasm), which again are made up of just four different nucleotides - A, C, G, and U. How these nucleotides code for amino acids was a point of contention between scientists for many years, with Francis Crick (co-discoverer of the structure of DNA) arguing for a complex and overlapping two-nucleotide system. The truth is much simpler. The four nucleotides can combine to form various three-letter arrangements (sixty-four, to be exact) known as codons. One codon codes for a "start" codon, three codons for a "stop" codon, and the remaining codons for the other amino acids. The more eagle-eyed quizzer will see that the numbers don't quite add up (sixty codons, but only twenty amino acids?). In fact, the "start" codon codes for the amino acid methionine. The remaining nineteen amino acids share the fifty-nine remaining codons, with some amino acids being coded for by more than one codon (arginine alone is coded for by six codons!).
The ribosome is an interesting beast. It is a small but complex machine of the cell which firstly recognises the mRNA codons, and then recruits molecules known as tRNA (the "t" here stands for transfer) which deliver the appropriate amino acids. It consists of a "large" subunit and a "small" subunit, which work together poetically to create proteins from mRNA codes - a process known as translation. Ribosomes are a mixture of proteins and nucleic acids, and it came as somewhat of a shock to the scientific community to realise that it was the nucleic acid (and not the protein) which carries out the catalytic activities of the ribosome. This finding, deduced from detailed structural analysis of ribosomes, earned Venkatraman Ramakrishnan, Thomas A. Steitz and Ada E. Yonath the 2009 Nobel Prize for Chemistry.
