The nucleus is only found in eukaryotic cells. It contains most of the genetic material (the DNA) of the cell. The genetic material of the nucleus is like a set of instructions.
The nucleus is surrounded by the nuclear envelope, a double membrane (two phospholipid bilayers) that controls what goes in and out of the nucleus. The nucleus also has holes embedded in the nuclear envelope. These holes are known as nuclear pores, and they allow things to flow in and out of the nucleus.
The nucleus of many cells also contains a central region called the nucleolus. The job of the nucleolus is to build ribosomes. These ribosomes flow out the nuclear pores into the cytoplasm. Ribosomes are organelles that make proteins in the cytoplasm.
The nucleus contains the genetic material of the cell.
The genetic material of the cell is found in chromosomes, DNA wrapped around proteins.
The nucleolus, which makes ribosomes, is also within the nucleus.