An organelle is a specialized structure within a cell that performs a specific function, much like an organ in a body. Organelles are found in both prokaryotic and eukaryotic cells, with eukaryotic cells containing more complex organelles such as the nucleus, mitochondria, and endoplasmic reticulum. Each organelle plays a crucial role in maintaining cellular processes and overall cell health. They are essential for functions like energy production, protein synthesis, and waste management.