The Monitors are a multiversal race charged with the protection of the Multiverse. During the first multiverse, there were only two, the Monitor and the Anti-Monitor, who oversaw the multiverse and the anti-matter universe respectively. Both were killed during the events of Crisis on Infinite Earths.
After the creation of the 52 in the new multiverse, a new race of 52 different Monitors is seen to be active trying to protect the multiverse from corruption as a community. Each monitor is assigned to its own world, and they all vary at least slightly in appearance. A version of the Anti-Monitor is also seen to be active helping the Sinestro Corps on Qward. Although they are seen again, the Monitors play their next biggest roles in Countdown.
Controversy emerges for the Monitors when two with radically opposing viewpoints argue about how the multiverse should be maintained, the chief concern being the presence of "Multiversal Anomalies" (people who have hopped from world to world). Monitor Solomon of Earth-8 believes such disturbances should be immediately eradicated, and goes as far as to hunt down and kill Duela Dent. Despite his violence and zeal, he manages to convert almost all of the Monitors to his way of thinking. However, New Earth's monitor, Bob, believes a much more peaceful approach should be taken, and establishes the Challengers from Beyond, a group made of anomalies to help him track down Ray Palmer who he believes is the only man capable of averting the upcoming Great Disaster.
The violent actions of the majority of the Monitors, including the genocide of the Forerunners, attracted the ire of Monarch, who believed them to be tyrants. Using the Bleed as his headquarters, he gathered an Army from all over the multiverse to challenge and destroy them, almost suceeding, culminating in the destruction of Earth-51. Monarch's whereabouts are currently unknown, all of the Monitors survived the Monarch War, except for Bob who was specifically killed by Solomon as Solomon attempted to directly absorb Bob and amass Bob's power for himself.
Although the Monitors were originally very close (some were specifically female while the bulk were male gendered), they have been changed drastically developing individual personalities and choosing through mutual best interest rather than a resulting singular mental/intellectual focus as shown when their base individual realities were drastically changed by Mister Mind during the end portion of the interdimensional 52 event. Thus the Monitors have also , they also changed to reflect them, and have since been becoming more and more individual and acting in mannerisms that would be considered individual greed of their particular reality and at times of their own individual person which resulted in the attack and removal of Solomon from their collective assembly.
Recognizing that the Monitors are more or less now completely individuals and thus increasingly dangerous to the stability of other individual realities, the Monitor of the destroyed Earth-51 Nix Uotan has recently put together a team to monitor the Monitors . The Monitor Guardian team consists of Ray Palmer, Donna Troy, Kyle Rayner and Forager. Recently Nix was sentenced to life on New Earth as a human, because of his failure to save Earth-51, though many of his peers felt the punishment unjust. Due to the mecahnitions of the New Gods, he was able to regain his powers, and has become something new. Though what the re-birth of the New Gods, especially Darkseid means for fate of the Monitors and the Multiverse remains to be seen.
it is revealed in Justice League #27 that the Monitors were splintered aspects of the original Monitor, but have since merged back into one singular being.
Smallville Season 11
The Monitors appear in the comic continuation of the show but unlike their mainstream counterparts, they are actively seeking the destruction of the Multiverse.