A cell is the smallest structural and functional unit of an organism. That statement is one of three parts to cell theory. Cell theory states that all living organisms are made from cells. All cells come from pre-existing cells. And the cell is the basic structural unit of living things.
Living things could be divided into 2 broad cell categories. Unicellular organisms (bacteria, many protists, some fungi) or multicellular organisms (lions, tigers, bears -- oh my). A multicellular organism (like a human) can be paired down into smaller and smaller chunks. An organism is made of organ systems, which are made of organs, which are made of tissues, which are made of cells. That's the final functional unit. Sure, cells have things like endoplasmic reticulums and golgi bodies, but those things do not exist on their own outside of cells. The cell is the smallest and simplest unit of living things.
Cells themselves can be subdivided into 2 broad categories. Prokayrotic cells or eukaryotic cells. Eurkaryotic cells have a nucleus and cell organelles (cell organs). Prokaryotic cells do not have organelles or a nucleus. They are basically an outer cell membrane, a cytoplasm, and some free floating DNA.
A cell is the smallest unit of life. Meaning that not only is it the smallest part of an organism, it is the smallest functional unit of an organism. Usually consisting of various organelles within it that help to maintain function, including: Nucleus, mitochondria, endoplasmic reticulum, golgi body, ribosome, and others.
A cell is the basic structural unit for all life. Cells are made up of various materials to help keep the organism alive. Cells have organelles such as the nucleus and the mitochondrion that perform different functions for the cell.
You can have organisms that are single cellular and organisms that are multi-cellular. There are two domains of organisms based on the cell structure. You have prokaryotes which have no defined nucleus or eukaryotes which have a defined nucleus. Eukaryotes are then further divided into animal cells and plant cells based on the makeup of the cell.