Taxonomists disagree on how to classify algae. They are sometimes classifies as protists, sometimes as plants, and sometimes divided between the plant and (former) protist kingdoms. Do not confuse eukaryotic algae with blue-green algae (now called cyanobacteria), which are prokaryotes.

Although both eukaryotic algae and cyanobacteria are important as primary producers in many environments, they are generally not of medical significance. (However, cyanobacteria of the genus Prototheca, which have lost their chlorophyll, have been reported to cause skin lesions.) Agar, which is used extensively in the microbiology lab, is a product extracted from smaller seaweeds (red algae). Some algae, such as kelps (brown algae), are used as food and in the manufacture of products such as cheese spreads, toothpaste and mayonnaise. They also enable beer to retains a foamy "head".