Ribosomes are large, complex molecular machines found within all living cells that serve as the primary site of biological protein synthesis. Ribosomes link amino acids together in the order determined by messenger RNA molecules. Ribosomes consist of two major subunits. The small ribosomal subunit reads the RNA while the large subunit joins amino acids to form a polypeptide chain. Ribosomes are found in both prokaryotes and eukaryotes. They are made of proteins and three or four RNA molecules called subunits.
Ribosomes can bind to and RNA chain and use it as a template for determining the correct sequence of amino acids in a particular protein. When the cell needs to make proteins you look for ribosomes because they are the protein builders or the proteins synthesizers of the cell. They connect one amino acid at a time and build long chains. Ribosomes are found in many places around the cell. They float inside the cytoplasm and others are found on the endoplasmic reticulum.
Sources cited: biology4kids.com, biology.About.com