Ribosomes are tiny structures inside cells that are involved in the production of proteins. Ribosomes are composed of ribosomal RNA--rRNA and proteins. Ribosomal RNA is produced by an organelle inside the nucleus known as the nucleolus. Cells which contain high numbers of ribosomes also manufacture a large amount of protein.
One place where ribosomes are observed in a cell is the cytosol. These ribosomes are known as free ribosomes.
Other ribosomes are bound to the outside of the endoplasmic reticulum or on the nuclear envelope. The endoplasmic reticulum that has ribosomes on its surface is called rough endoplasmic reticulum.
Free ribosomes are involved in the production of proteins that will remain in the cytosol. The bound ribosomes synthesize proteins that will enter the endomembrane system to be transported and then secreted outside of the cell so they can be transported to where they are needed. Bound ribosomes are also responsible for making proteins that will become part of the cell's various membranes or as components of cell organelles.
I have included a link showing rough e.r. with its ribosomes from an electron microscope image.