Nucleolus

A nucleolus (plural: nucleoli) is the part of a eukaryotic cell where ribosomes are made. Seen under a microscope, the nucleolus is a dark spot inside the cell's nucleus. It is made up of dense RNA and proteins. Prokaryotic cells also have ribosomes, but they don't make as many as eukaryotes do, and there is no visible nucleotatius. There is usually more than one nucleolus in a eukaryote cell. The nucleolus is not bounded by a membrane. 
The nucleolus is made at a nucleolus organizer region (NOR), which is a chromosome region around which the nucleolus forms. Inside the nucleolus ribosomes are made. Ribosomes are molecular machines that synthesizes proteins. The ribosomes are exported through the nuclear envelope to the cytoplasm, where they do their work.
Nucleoli play a role in the cell's response to stress. They also help in the assembly of signal recognition particles. Malfunction of nucleoli can be the cause of several human diseases.