You're describing algae, which are a diverse group of single-celled or multicellular eukaryotic organisms that are not classified as plants. While they do perform photosynthesis like plants, they lack the complex tissue structures and organs found in plants.

Here's a breakdown of algae:

* Microscopic and Single-celled: Many algae are single-celled, known as microalgae, and are only visible under a microscope.

* Harmful and Helpful: Algae can be both harmful and helpful:

* Harmful: Some algal blooms can produce toxins that are harmful to humans and animals.

* Helpful: Algae are a vital part of the ecosystem, producing oxygen and serving as a food source for many aquatic organisms. They are also used in various industries like food, cosmetics, and biofuel production.

* Classified by Shape: Algae are classified based on their shape, pigments, and other characteristics. Some common shapes include:

* Diatoms: These are single-celled algae with intricate silica shells.

* Dinoflagellates: These are single-celled algae with two flagella for movement, some of which can cause harmful algal blooms.

* Green algae: This group includes a wide variety of forms, from single-celled to multicellular, and are closely related to plants.

* Red algae: These algae are typically multicellular and contain pigments that give them a reddish color.

* Brown algae: This group includes large, multicellular seaweeds.

Important Note: It is crucial to remember that algae are not plants. They are a distinct group of organisms with unique characteristics and classifications.