There are large numbers of ribosomes in all cells, and can be found either free in the cytoplasm, or attached to the rough endoplasmic reticulum.

  They are made from ribosomal RNA, 65%, and protein, 35%.

  They are the sites of protein synthesis

  Protein synthesis is done by assembling amino acids into chains in the right order, shown by the code on messenger RNA.  

        In general, free ribosomes make proteins to be used inside the cell, whilst proteins bound to the rough endoplasmic reticulum make proteins to be excreted out of the cell.

A ribosome - single amino acids are entering, and becoming amino acid chains - proteins.