Unicorns are notably identified by the horns on their heads, mostly shown as long and spiral-like as in traditional unicorn species of greek mythology. A unicorn can appear just as varied as the other races, though all kinds of unicorns do hold their signature horn.
Out of the three main species, Unicorns are the most strongly connected to magic, and has shown to successfully perform numeous amounts of spells and abilities throughout their appearances, most if not all of their magic revolving around their horn. Before the founding of equestria, and the birth of Celestia and Luna, it is implied that Unicorns can use their magic to such a level that any unicorn can mvoe the sun and moon, and as such move the days forward. This, much like the weather control from the Pegasus, are used as bribary for the Earth Ponies prior to Equestria's founding. The most common ability used for Unicorns is a telekinetic magic, in which they wrap a magical aura around and object and as such capable of lifting and moving it off the ground. Most spells need a line of sight for both activation and deactivation.
Despite all of their magic, a unicorn's magic is mostly connected to their horn, and is possible to slow down, or even halt a spell used by a unicorn by covering, damaging, and/or removing the horn. The most notable depiction of this manor was shown when Sombra removed Twilight Sparkle's horn, and as such making her unable to use her magic against him during her capture.
While found throughout Equestria, more notable places for unicorn inhabitants are in more high-class areas, such as Canterlot.