DNA, abbreviation of deoxyribonucleic acid, is the chemical present at the centre of the cells of living things, that controls the structure and purpose of each cell and carries genetic information.
The cell can be compared to restaurant and the differentiated cell types are like restaurants specializing in different cuisines. The DNA is the set of instructions for the cell, just like a cook book which is the set of instructions for a restaurant. A single DNA instruction is a gene which is akin to a single recipe in a cook book.
RNA polymerase is like a photocopy machine and transcription is the process of making an RNA copy of a gene. The mRNA transcript (i.e. photocopied recipe) is transported to the ribosome, which functions as the cook. The ribosome reads the mRNA one codon at a time, as a cook would read the recipe one step at a time. Each codon corresponds to an amino acid just like each step in a recipe refers to a specific ingredient. The amino acids are brought to the ribosome by tRNA. These tRNA molecules are like kitchen hands. The ribosome then joins the amino acids together to make a polypeptide (protein) which can be compared to a cook mixing the ingredients to make food.