Protists are a large and diverse group of eukaryotic microorganisms, which belong to the kingdom Protista. The term Protoctista is also used for these organisms. Molecular information has been used to redefine this group in modern taxonomy as diverse and often distantly related phyla. The group of protists is now considered to mean diverse phyla that are not closely related through evolution and have different life cycles, trophic levels, modes of locomotion and cellular structures. Besides their relatively simple levels of organization, the protists do not have much in common. They are unicellular, or they are multicellular without specialized tissues; this simple cellular organization distinguishes the protists from other eukaryotes, such as fungi, animals and plants, although some fungi and animals are also unicellular.
Protists were traditionally subdivided into several groups based on similarities to the "higher" kingdoms: the unicellular "animal-like" protozoa, the "plant-like" protophyta (mostly unicellular algae), and the "fungus-like" slime molds and water molds. These traditional subdivisions, largely based on superficial commonalities, have been replaced by classifications based on phylogenetics (evolutionary relatedness among organisms). However, the older terms are still used as informal names to describe the morphology and ecology of various protists.