Algae are a diverse group of photosynthetic organisms that can be unicellular or multicellular. They are classified into three main types - green algae, brown algae, and red algae - based on their pigmentation and characteristics. A comprehensive classification system proposed by F.E. Fritsch divided algae into 11 classes according to their structure, reproduction methods, and other traits. Algae are found in many environments and have various uses including in food, cosmetics, and industry. They are important primary producers and indicators of water quality.