What is nucleus where it is found?

Nucleus :  The nucleus is present only in eukaryotic cells. It is the control center and is generally found in the center of the cell.
The nucleus is the site of synthesis of the molecules responsible for transmission of genetic information - DNA, or deoxyribose nucleic acid, and RNA, ribonucleic acid. A double membrane called the nuclear envelope surrounds it.

Outside the nuclear envelope is a gel-like substance called the cytoplasm, which fills most of the cell. The cytoplasm contains organelles and various types of filaments that serve to hold the shape of the cell and become involved in cell division, locomotion, and adhesion. RNA and other substances move from the nucleus to the cytoplasm through complex holes in the nuclear membrane called nuclear pores.

The counterpart of the cytoplasm inside the nucleus is the nucleoplasm, which contains fine strands of chromatin, a combination of DNA and proteins. During cell division, chromatin coils up and forms chromosomes.

Most nuclei also contain one or more spherical or egg-shaped structures called nucleoli (singular - nucleolus). The function of a nucleolus is to synthesize and partially assemble ribosomes, structures that serve in the assembly of proteins. The ribosomes then migrate to the cytoplasm where proteins are formed.