A cell is the basic unit of living organisms that is capable of reproducing itself exactly. That is, although eukaryotic cells (such as plant cells and animal cells) contain components, which are called organelles, those components do not reproduce themselves independently of the cell of which they are a part.
The nucleus is the "Control Center" of each cell, which contains DNA (deoxyribonucleic acid, which is genetic information) in the form of genes, and also information for the formation of proteins.
DNA, which is combined with protein, is normally dispersed throughout the nucleus as chromatin. During cell division the chromatin becomes visible as chromosomes. The nucleus also contains ribonucleic acid (RNA), most of which is located in the nucleolus.
See also the functions of cell organelles.