While the nucleus is home to cell's genetic material, the nucleolus contains developing ribosomes. The nucleus is surrounded by a porous double membrane, and the nucleolus is made of ribosomal RNA proteins.
The nucleus is responsible for many of the cell's processes and carries its essential information, and the nucleolus rests inside it. Inside the nucleus is everything the cell needs to fill its role. Around the outside, there is a porous membrane, which allows for the passage of ions, water, ATP for energy and other micromolecules. The DNA inside the nucleus remains inside, but it can transmit information outside the cell in the form of mRNA. This makes reproduction and other cellular processes possible.
In contrast, the nucleolus has a very specific task, which is manufacturing ribosomes. Like the nucleus, it has a small role in mediating cell signaling processes and is particularly useful when responding to cell stressors. Unlike the nucleus, it does not feature a cell membrane. In most human diploid cells there is one nucleus, although as many as 10 appear during mitosis. The matrix is not well organized. It is made primarily of rRNA molecules, so it is not as diverse as the nucleus.