Algae (one alga, but several algae) are a type of plant-like living things that can make food from sunlight by photosynthesis. The study of algae is called phycology or algology.
The term lumps together many different kinds of organisms. They have in common only that they are autotrophic: they use natural sources of energy and simple inorganic materials to build their forms. As non-vascular plants they do not have the kind of cell and tissue structure of land plants. They are a convenient but very loose term. Only in recent years has it become clear how different the many kinds of algae are.
Algae are a large and diverse group of simple, typically autotrophic organisms. Some have one cell and others have many cells. The largest and most complex marine algae are called seaweeds. They are like plants, and "simple" because they lack the many distinct organs found in land plants. For that reason they are not classified as plants.

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