All protists are eukaryotic, meaning they feature a cellular structure with nuclei to contain their genetic material. Also called protozoans, which means "first animals," all protists prefer a moist environment and are found where there is perpetually moist soil or in freshwater and saltwater bodies of water.
Along with the nucleus, most protists also feature mitochondria which aid in metabolic functions, and vacuoles which aid in digestion. This means that despite sometimes being of very small size, protists are relatively complex organizations from a functional standpoint.
Other than these basic similarities, the nature and features of protists vastly vary. They may be unicellular or multicellular and range in size from microscopic to 300 feet in length. However, even in the largest multicellular protists, there is no cellular specialization or differentiation in tissues types. While protists are not classified as plants, animals or fungi, they may display plant-like, animal-like or fungi-like characteristics.
The Kingdom Protista was established by the scientist Ernst Haeckel in 1866. It was not officially recognized by the larger scientific community until a century later in the 1960's. The Kingdom Protista is often called a"junk drawer" since it comprises all the eukaryotics that cannot be classified into any other kingdom.