Algae are regarded to be in the kingdom protoctista, which is a conglomeration of organisms that do not fit into the plant, animal, or fungi kingdoms. Most algae are unicellular or multicellular eukaryotes that contain chlorophyll that is used to make their own food via the process known as photosynthesis. Most algae live in aquatic or moist environments. Examples of algae include diatoms, kelp, and spirogyra.
Because of the range of diversity amongst algae, a scientist by the name of Fritsch attempted to classify algae into eleven classes in 1935. He classified algae based on their types of pigments, the methods of locomotion that are used by the various algae, the various ways in which the algae reproduce, how algae reserve food, etc. The eleven classes of algae are:
Characteristics for each of the eleven classes of algae can be found here.