The Overlookers were an alien species from the Delta Quadrant. The overarching polity or sociopolitical entity to which the species belonged and comprised was called the Hierarchy. Their species was highly adept in the skills of espionage and remote surveillance.
Physically, the species was humanoid with dull brown leathery skin and generally nondescript features. Their heads were lumpy and hairless, with few distinguishable facial features, having no eyebrows, lips, or nose – though nostrils were present. Their ears were similarly understated, marked only by subtle embossing around curved openings on the sides of their heads. There also appeared to be two vertical rows of at least eight raised pits running down the back of their heads. Additionally, members of the species possessed a rotund appearance with a relatively nonathletic build compared to other humanoid species.
They regularly used surveillance on passing ships to plan future raids. This was done by special surveillance ships, hiding in nebulae for example, which used highly advanced (compared to other species) sensors to assess the tactical risk of attacking passing vessels and the value of their cargo. The Hierarchy's cloaked ships also allowed them to sneak up on targeted vessels, after which they would execute a predetermined attack pattern, such as the type 3 or type 4 stealth assault. Their attack patterns were numbered according to their increasing levels of energy expenditure. The Hierarchy was meticulous when it came to the use of energy and resources on its vessels. Every planned use was to be reported and consulted with the Hierarchy to maximize efficiency. (VOY: "Tinker Tenor Doctor Spy")
The Hierarchy itself was regularly consulted by crews of its ships using a special communication system. The situation was detailed on a sort of PADD, then entered in a slot next to a cylindrical horizontal display. Changing colored patterns would subsequently appear on the display to indicate the Hierarchy's decision.
The social and working ethics of the Hierarchy differed dramatically from those of Starfleet in that every crewmember had only a single duty and workstation, minimal social interaction, and limited access to the rest of the ship. However, some members of the Hierarchy were known to leave their territory and explore other areas of space (VOY: "Renaissance Man"), and the Hierarchy, while omnipotent, was not necessarily appreciated by all Overlookers: Phlox, in particular, befriended The Doctor after getting to know him; he came to admire how he had evolved past his original condition – something that was beyond reach for a member of the Hierarchy (VOY: "Tinker Tenor Doctor Spy") – and went as far as warning the Voyager crew of the impending Hierarchy attack to prevent their destruction. Similarly, Nar came to sympathize with Captain Janeway during her detention, and willingly knocked down his superior to allow her to escape (VOY: "Renaissance Man").
A Hierarchy survey vessel was one of the many that got trapped within the subspace anomaly known as the Void. The Hierarchy crew joined Captain Janeway's alliance, where they uncovered an imminent threat with their espionage techniques. (VOY: "The Void")
Hierarchy starship classes
The species was not named on screen but the reference book Star Trek: Voyager Companion (p. 329) described them thus; "These are high tech 'spies', but nobody here is James Bond... they're more like office workers... cogs in a vast, alien bureaucracy that we will come to know as the 'Hierarchy'. In later episodes 'The Void' and 'Renaissance Man', the shooting scripts refer to them as 'Overlookers' and 'Overlooker aliens' respectively."
Specifically, the script for "The Void" describe them as "the hefty, nervous species known as "the Overlookers."
The call sheets for the episode "Renaissance Man", dated Thursday 15 March 2001 and Friday 16 March 2001, mentioned the interior of the "Overlooker Ship" (built on Paramount Stage 16) and listed Zet and Nar as "Overlookers" in parentheses.
The costume worn by Andy Milder and Mike Satterfield features a costume tag which reads "Andy Milder, #270 Renaissance Man, Overlooker Nar" and "224 Tinker, Tenor, Doctor, Spy, ND, Mike Satterfield, Overlooker". 
The Overlookers appear in the Delta Rising expansion of the online game Star Trek Online, though they are simply called the "Hierarchy", even as a species. The Hierarchy are portrayed as highly adept in intelligence-gathering, surveillance, and combat support, and can join the player as allies during a conflict with the Vaadwaur. One of the storyarc's mission also allows the player to obtain an Overlooker Science Bridge Officer (in-game referred to as a Hierarchy Science Officer).