The nucleus of a cell is home to what makes you unique, your DNA. Its function is to control gene expression and help copy the DNA when your cells grow and divide. The nucleus looks like a ball in the centre of a cell.
Not every cell has a nucleus. Bacteria for example lack a nucleus for their DNA and red blood cells kick out their nucleus (and DNA) when they mature. Most cells have one nucleus, but some can have many as a result of cells fusing together. In the human body, your muscle cells gain multiple nuclei when they fuse together during development.
The nucleus is wrapped in a double membrane (has two layers) that is supported by a variety of proteins including ones called Lamin’s. These proteins act as a scaffold to hold the membrane in place. When Lamin’s are made incorrectly, they cannot support the nuclear membrane. This makes it difficult for the cells to grow and divide properly and results in diseases like progeria.
Your DNA is stored in the nucleus in the form of a chromosome. Chromosomes are DNA wrapped around spool like proteins called histones. Inside the nucleus, DNA is copied into RNA in order to make the proteins your cells need to survive. This is a tightly controlled process since any damage to the DNA could result in an error in the message. As such, entry into the nucleus is controlled by gates or pores on the membrane. The gates only allow certain proteins into the nucleus. Also within the nucleus, there is a region where the ribosomes of your cell are manufactured called the nucleolus.
The nucleus acts as both a command centre of the cell and the security deposit box or safe of your DNA.