Nucleus: Nuclear Envelope, Nuclear Pores, Chromatin, and Nucleolus

Nucleus:
Contains nearly all the cell's DNA and with it the coded instructions for making proteins and other important molecules.

Nuclear Envelope:
Double lipid bilayer membrane which protects the DNA and keeps it located inside the nucleus.

Nuclear Pores:
Small hollow tubes on the nuclear envelope which allows material like RNA to move in and out of the nucleus.

Chromatin:
DNA bound to protein. The structure depends on the stage of the cell cycle. During interphase, chromatin is loose threads. When the cell divides, chromatin condense into chromosomes.
Chromatin packages DNA into a smaller volume to fit inside the nucleus and prevent DNA damage.

Nucleolus: A small dense region of the nucleus. It is where the assembly of ribosomes begin.