Plastids are specialized organelles found in plant cells and some protists, playing crucial roles in the synthesis and storage of food. They include various types, such as chloroplasts (involved in photosynthesis), chromoplasts (which contain pigments), and leucoplasts (involved in storage). Plastids are characterized by their double membrane and their own DNA, allowing them to replicate independently within the cell. They are essential for processes like energy production and the storage of starch, oils, and proteins.