Euphausiids are small shrimp-like crustaceans. Like other eucarids, they have a carapace (carapace) fused to the entire thorax and have stalked, compound eyes. Their basic body plan includes 5 cephalic, 8 thoracic, and 6 abdominal segments. The first 2 body regions are fused as a cephalothorax (cephalothorax). Their cephalic appendages consist of paired 1st antennae with peduncles and 2 flagella, 2nd antennae with one flagellum and scale (squame), and the following mouthparts: labrum; mandibles usually with palps; labia; and 1st and 2nd maxillae. The relatively small size of the exopod on the 2nd maxilla is thought to be characteristic of Euphausiacea. They bear 8 pairs of thoracic legs (thoracic legs) which are similar in shape and function in the genus Bentheuphausia , but variously modified in other genera (Thoracic leg 2 (N. difficilis)); see the discussion of phylogenetic relationships in Higher Taxa). The thoracic limbs bear large, feathery gills (gill detail ,E.pacifica gills, eye, & lappet) on the coxae. The first 5 abdominal segments bear pleopods (pleopod , Pleopod motion (E.pacifica)) and the 6th segment bears uropods and a telson (tail fan (juv. T. spinifera)). Most species have photophores, typically arranged in pairs on the eyestalks and bases of thoracic limbs 2 and 7, and as single photophores located mid-ventrally on abdominal segments 1-4 (photophores ventral photo). Euphausiids lack statocysts.

The term "euphausiid" is commonly used for all members of the crustacean order Euphausiacea, which contains the two families Euphausiidae and the mono-specific Bentheuphausiidae. Although the ending -id , as in euphausiid, correctly applies only to members of a given family, e.g. Euphausiidae, in common usage it also includes the one species of the second family.

The word Euphausia derives from Greek eu for good or true, combined with -phausia for shining or light emitting. Early naturalists were impressed by the brightness of the photophores of these small animals (Bioluminescence 1 (spiraling) , photophore flash pattern). Another term, krill, has become synonymous with euphausiid. Krill was first used in this sense by Norwegian whalers who applied it to the swarming little fish (krill) which signaled whale feeding grounds. A whale may consume many tons of North Atlantic or antarctic krill in a day.

Larval euphausiids to 4 or 5 mm body length are considered true components of the zooplankton, being dispersed primarily by water circulation. Maturing euphausiids become increasingly capable of directional migration and are sometimes designated micro-nekton. However, this distinction is frequently ignored in analysis of zooplankton net-samples which, indeed, retain substantial proportions of adults, particularly if collected at night.