Chloroplasts are organelles that contain chlorophyll, the green pigments that make plants green. They are often called plastids, though plastids and chloroplasts are not exactly the same thing—a chloroplast is a special kind of plastid that photosynthesizes. Chloroplasts do lots of things, but their main function is photosynthesis.
Plant cells, and some protists have chloroplasts, though animal and fungal cells lack them. Prokaryotes do not have chloroplasts (or any other organelles), though some can carry out photosynthesis, its cell acting like one big chloroplast. In most cases, chloroplasts are green.
Chloroplasts are cellular generators. They take in carbon dioxide and water, and release sugar and oxygen. This process is called photosynthesis. Mitochondria then use the sugar and oxygen to carry out cellular respiration, producing energy.
Mitochondria are essential parts of many eukaryotes, but they are useless without oxygen. Therefore, chloroplasts are extremely important. They produce breathable air for life.
Like mitochondria, chloroplasts have their own DNA. Scientists also think chloroplasts are descended from a kind of bacteria, called cyanobacteria. Cyanobacteria are the photosynthetic bacteria we mentioned earlier. In early stages of Earth, cyanobacteria produced oxygen for their friends, the ærobic bacteria (ærobic means they use oxygen). Some of these ærobic bacteria later migrated into a eukaryotic cell, becoming mitochondria. The cyanobacteria followed, becoming chloroplasts. All eukaryotic cells contain mitochondria, but only plants and algæ contain chloroplasts, so scientists think mitochondria came first.