The oldest of the Citadel races, asari are a beautiful (by just about all Citadel species standards) mono-gendered race of humanoids with skin tones in widely varying shades of blue and faces with ridges that resembles a starfish or squid. Commander Shepard was notable for recruiting two or more asari for their teams (the first being Liara T'soni, the next being either Samara and possibly later her daughter Morinth, and finally Liara once more).
According to Liara when prodded into a response from Joker during the Reaper War, the "hair tentacles" of the asari are semi-flexible cartilage-based scalp crests that grow into shape, which do not flop around.
Asari are a very long-lived species, capable of living up to a thousand years or more throughout the three stages of the lives and possessing by the end of it a great store of experience and wisdom that all asari (and most races) respect. Most asari are very good biotics, possessing skill and power that most races can't come close to without intense training and biotic amp implants. In fact, one of the conditions for an asari to become a commando is to possess a certain degree of biotic power (enough for it to be used practically, in other words, combat).
Asari reproduce from a form of parthenogenesis, through a process of "melding" during which the Asari attunes her nervous system to her partner's, during such a session the Asari and her partner temporarily become one unified nervous system, sharing thoughts, emotions, and even feelings (such as physical pleasure) this can be done for a variety of reasons (sometimes as simple as the transfer of information), but when used for mating the Asari provides two copies of her own genes to her offspring, one of which is randomized from the genetic template of the other parent.
Due to this some Asari believe that their offspring inherit certain genetic traits of the "father", though evidence of this is purely anecdotal at best. This unique form of reproduction not only allows the asari to "mate" with just about any species including the Krogan, Turians, Salarians, Humans and possibly even Volus and Quarians, but is believed to be why they are such accomplished biotics. They primarily do this with other species as to "bring about understanding..." of the other races, believing that through this the asari species grows and learns more of their ally and friend species. However, asari can reproduce with another member of their race, but to do so is, by some, considered akin to a taboo.
Asari believe that "pureblood" asari do not contribute to their race and thus the word pureblood is considered a grave insult, though this is rarely spoken to a purebloods face, which is Ironic considering that before they achieved space travel they had no choice but to mate with their own species. Though arguably one of the most humanoid races in Mass Effect, a bachelor party on Illium revealed to Shepard that humans, salarians and even turians believe that asari resemble their race, with the human member of the party humorously asking, "Do you think, you know, the Asari are like, controlling our minds?"
Asari, being long lived as they are, have three very different life stages, akin to humans except altered to their long lifespans. First is the "Maiden" stage, which could possibly be considered to be the "teenage/early adult" years (or centuries in their case). During this stage young asari girls have an almost insatiable thirst for adventure and danger, many taking up a profession as pirates, mercenaries and exotic dancers.
This eventually gives way to the "Matron" stage believed to be around 300-350 "years" of age, though this can occur sooner if the Asari "melds" frequently. This stage is marked by a desire to quit wandering, settle down, and raise a family.
After this begins the "Matriarch" stage around 750 "years" or so, or sooner if the individual has melded rarely (polar opposite of the matron requirement). By this point most Asari have accumulated a vast wealth of knowledge and experience that is highly valued to their people, Because of this the Matriarchs are considered to be "wise women" of the Asari people, and are deferred to when important decisions need to be made. Because of this Asari have a "longview" not common in other species, in fact its quite normal for an Asari (especially Matriarchs) to make decisions or investments that at the time seem ludicrous, only for such decisions to pay off as the Asari predicted decades or even centuries later. However this is not always the norm, as not all maidens choose to wander, and some matrons do not wish to become mothers, also not all Matriarchs choose to become leaders or advisors.
According to in game canonical references, hailing from their homeworld Thessia, the Asari were the first species in several millennia to discover The Citadel and use of the Mass Relays spread across the Milky Way. They were also the first to suggest a cooperative government between the race of the Turians, Salarians and Asari to regulate commerce and inter-species politics that might well lead to war or tension. In politics, and in some outside cases, Asari play a neutral ground in conflicts and debates, trying to promote peace and cooperation between two or more factions. The Asari homeworld of Thessia is unusual as compared to most other races, in several ways. Firstly, the Asari homeworld is supposedly divided into several republican city-states (reported to be much like the ancient Greek city-states of Earth) who while independent still maintain close ties to one another.
The Asari also never fell prey to civil war and infighting (as species like the Turians and Humans did), instead choosing to open diplomatic ties and trying to understand one another through trade and diplomacy, leading to a unity not often seen in species who had yet to leave their homeworld. The Asari also possess no real military to speak of aside from their rather formidable navy, instead the typical Asari military unit is small bands of commandos from each City on their homeworld who are organized and equipped at the local level (each Asari arming and equipping herself). Commandos are trained, hired, and equipped by the cities they hail from starting at a young age (most commandoes are said to be in the early stages of the maiden phase of their lives, and that by the time they retire from commando duty they posses an alarming proficiency for killing), and depending on the size of a commandos respective city or town her band can range from a few small part-time volunteers with only the most basic of equipment, to larger more professional forces from the larger cities.
These bands however are often quite large and operated by full-time professionals with the latest weapons and equipment. Asari Commandos are said to usually fight single or in pairs, It is also rumoured (though yet to be proven valid) that commandos are the best soldiers in the galaxy and are more than a match for most infantry units. However The Asari Commandos are not professional frontline troops and they cannot wage a typical war campaign nor can they stand up to a firefight the way Humans, Krogan, and Turians can, they just don't have the numbers. Instead they rely on assassination, sabotage, espionage, etc. to achieve their aims.