Cells are the basic unit of structure and form in all living things. Some organisms are made of only one cell (unicellular) while others are made of trillions (multicellular). Each cell must be able to carry out the basic functions of life, including obtaining and using nutrients, producing proteins, shipping said proteins, etc. Some cells (prokaryotes) are relatively simple with very few internal structures. They manage, in part, because they are so small that it does not take much coordination to get things done. Larger and more complex cells (eukaryotes) require specialized structures to accomplish all of the necessities of life. These structures are called organelles, and they include the nucleus, Golgi apparatus, endoplasmic reticulum, vacuoles, mitochondria, and more.
If you were to compare a eukaryotic cell, which includes plants and animals, to a factory, you might see the ribosomes as workers (they produce protein), the nucleus as the boss (it controls what is happening in the cell), the rough endoplasmic reticulum as the conveyer belt (this is where the protein is produced), the Golgi apparatus as the shipping center (it packages and ships proteins), and the mitochondria as power generators (they use food to provide usable energy to the cell). Following this analogy, you should be able to come up with roles for the rest of the organelles within your factory.