The nucleus is the "brain" of the cell. Its job is to control the cell's structure, function, growth and division. It is usually located in the center of the cell, where it is enclosed in a membrane called the nuclear envelope – which like the cell membrane is also a semi-permeable barrier. The nuclear envelope is a double membrane as shown in the diagram.
The nucleus communicates with the rest of the cell through the nuclear pores. The nucleus is like a vault. It holds the cell's DNA -- which contains the genetic instructions for the cell's activities. In the diagram there is a structure within the nucleus called the nucleolus. This structure is the place where ribosomes are made. Ribosomes are very important structures upon which proteins are formed within the cell. In the diagram above, you can see the ribosomes that have already been sent out of the nucleus and are on the endoplasmic reticulum, one of the places within the cell where proteins are made.
One way the nucleus directs activities within the cell is by sending instructions through its envelope to the various organelles in the cell's cytoplasm. There are two main types of organelles -- organelles that produce protein and organelles that transfer chemical energy present in food molecules such as glucose into molecules called ATP (short for adenosine triphosphate). The chemical energy present in ATP can then be used by the cell for such activities as movement and building large molecules.