Species were unique lifeforms. They were generally divided into three classifications based on their level of sentience: sentient, semi-sentient, and non-sentient. There were over 20 million sentient species known to the galaxy. Sentient species were sometimes erroneously referred to as races, which more accurately referred to groups or subspecies within a species.
Species in the Star Wars universe generally have a large compatibility to real-world classification systems. However, these are not always strictly adhered to. For instance, according to The Essential Guide to Vehicles and Vessels, the Colossus wasps «follow the standard insect physiognomy, with a head, thorax and abdomen,» which corresponds with the real-world physiognomy of insects; however, the Sic-Six, which also have these three body segments but eight legs, are classified as arachnids, which in the real world have only two body segments. Star Wars also has numerous examples of species sharing characteristics from different classifications, such as the Quor’sav species. They have a combination of monotreme—mammals that lay eggs—and avian characteristics.