Their true name is a name given to them by an ancient race who were among the First Ones. The name "Shadows" was given to them by the younger races, as their true name is said to be over 10,000 letters long and unpronounceable by any of the younger races.
The Shadows are insectoid hexapeds that resemble a cross between an Earth spider and praying mantis with a small triangular head and seven pairs of pinpoint eyes in two stacked rows (3 above, 4 below.) Externally, they're colored in shades of dark purple and black and posses the ability to cloak themselves from most visible wavelengths. They can however sometimes be seen as figures of static in the upper end of the infrared spectrum. Shadows, like Vorlons, have evolved into beings of light, but unlike them prefer to assume material form.
Shadow speech sounds like a dynamic screeching that can be understood by their thralls, such as Morden, Anna Sheridan and Justin, through an implant