What is an organelle?
Think of an organelle as a "tiny organ" that is a component of a cell. The organelle usually contains a membrane which helps create a compartment where certain cellular activities can occur without interference from other elements in a cell.
What do ribosomes do?
Ribosomes are where RNA is translated into protein. This process is called protein synthesis. Protein synthesis is very important to cells, therefore large numbers of ribosomes are found in cells. Ribosomes float freely in the cytoplasm, and are also bound to the endoplasmic reticulum (ER).