Cell theory states that all living things are made of cells, which are the basic unit of structure and function. There are two main types of cells - prokaryotic and eukaryotic. Prokaryotic cells lack a nucleus and organelles, while eukaryotic cells have a nucleus enclosed in membranes and various membrane-bound organelles. The plasma membrane regulates what enters and exits the cell, and cells use various transport mechanisms like diffusion, osmosis, and active transport. Plant cells have cell walls and often chloroplasts, while animal cells lack these features.