Ribosomes are free particles within cells and also particles that are attached to the membranes of the endoplasmic reticulum. They are made up of about 40% protein and 60% nucleic acid. You can find more information here: http://www.historyoftheuniverse.com/ribo...

Ribosomes are complex of RNA and protein molecules. They are called protein factories because these are the sites of protein synthesis.