All living beings are made up of complex structures called cells. Cells are made up of billions of biomolecules working together. Viruses are not regarded as living beings because they are not made up of cells.
Living beings can be (a) unicellular: made up of just one cell (bacteria, protozoa, many algae, yeasts); and (b) multicellular: made up of more than one cell: (some algae, most fungi, plants and animals).
All cells are able to perform the three vital functions: (a) they reproduce, quite usually by mitosis, a process that yields two daughter cells with almost identical genetic material; (b) they interact with their environment, giving responses to specific stimuli, as when a white blood cell is able to detect and destroy a bacterium; and (c) they feed, meaning that they are able to exchange matter and energy with their environment, as when a human cell takes oxygen from the blood and releases carbon dioxide.
All the cells have (a) a cell membrane, which is the cellular envelope, (b) a cytoplasm with organelles, which are specialized cell compartments where specific functions are fulfilled, and (c) genetic material, that carries the instructions that allow both the cellular work and its self-construction.
There are two main kinds of cells: (a) the cells of the bacteria and archaea have no real nucleus: they are said to be prokaryotic; (b) the cells of all the other living beings (algae, protozoa, fungi, plants and animals) have their genetic material separated from the cytoplasm by a nuclear membrane: they are said to be eukaryotic.
The cells of the plants can be easily distinguished from those of the animals because (a) they have a semi-rigid cell wall, made of cellulose, surrounding the cell membrane, that usually gives the cell a polyhedral shape; (b) they have one kind of organelles, called chloroplasts, where sunlight energy is used to start building their own organic substances through a chemical process called photosynthesis; (c) they use to have one or a few big vacuoles that contain sap (instead of lots of smaller ones without sap) which normally push the nucleus out to the periphery of the cell; and (d) although they have equivalent structures, they lack centrosomes, the organelles that control the arrangement of the chromosomes during mitosis in an animal cell.