Polyribosomes or polysomes, occur in clusters on one mRNA molecule, where they carry on simultaneous translation.
Bound ribosomes are located along the rough endoplasmic reticulum (and on the plasma membrane of prokaryotes). Their polypeptides enter the ER.
Polypeptides destined to enter the ER's lumen are capped by a signal sequence which helps direct their entry. Once the polypeptide enters the lumen, its signal sequence is removed. As the polypeptide grows it binds the ribosome to the ER.
"Recent discoveries strongly suggest that the ribosomes studding the rough endoplasmic reticulum are at least partially held there by the specific polypeptide they are synthesizing. Note--it is the mRNA (in this instance) and not the ribosome that does the moving. At its leading (N-terminal) end, the polypeptide contains a signal peptide that will bind with a specific surface receptor protein in the membrane of the RER. Within the lumen, an enzyme called signal peptidase cleaves the signal peptide from the growing polypeptide, and the remainder can then fold into its active form taking on its normal function."
Free ribosomes are scattered throughout the cytoplasm and make proteins used within the cell.