It is a series of video games produced by Nintendo. One of the company's most notable franchises, the series' popularity spans several Nintendo consoles, with the first game, Metroid, released in 1986 for the Nintendo Entertainment System. Despite its popularity, it hasn't been the best in terms of sells, and has died twice as a franchise, once after Super Metroid, and another time after Metroid: Other M. Despite these shortcomings, it still remains as one of Nintendo's primary IPs.
The Metroid games chronicle the missions of bounty hunter Samus Aran in a science fiction setting which is heavily inspired by the Alien film franchise. Central plot elements are the Metroid organisms, Phazon, and the Space Pirates which try to exploit the Metroids' powers. The gameplay combines adventure based on exploration and item-gathering with platformer and shooter dynamics.
In terms of tech: The Galactic Federation reaches to about tier 8 and MFTL+ flight, the Space Pirates can break into Tier 5. Chozo and Luminoth tech hasn't been as explored, but it has shown a lot of power with what we've seen they can reach tier 4. Phazon amplifies said tech to absurd levels.
Characters: Space Pirates that are easily Town level, Dark Samus survived the explosion of Dark Aether, the Metroid can consume any kind of energy and grow stronger and deadlier, the Phazon mutates or kills living beings by screwing around with its genes, the X Parasite can assimilate everything related to their host (their look, powers and even knowledge) and they grow stronger each time. One of the haxiest, the Ruins Test is extremely dangerous, although ironically one of the weakest. Animus has the power to reset the universe through a massive black hole. Basically, this is a verse with the hax to back up the power.
Additionally, the lore of the verse boosts Phazon's hax to epic proportions, even including reality warping and anti-hax.
Metroid Zero Mission
Metroid Prime: Hunters
Metroid Prime 2: Echoes
Metroid Prime 3: Corruption
Metroid Prime: Federation Force
Metroid: Samus Returns
Metroid Other M
Metroid Prime 4 (Working Title)
Samus and Joey (Most likely takes place after Super Metroid considering there are multiple references to the events of that game in Samus and Joey)
Notable Non-Canon / Former Canon
Metroid (NES). Replaced by Metroid: Zero Mission
Metroid 2: Return of Samus. Replaced by Metroid: Samus Returns
Metroid: Zebes Invasion Order
Metroid Wanpakku Manga
Super Metroid Manga
Super Metroid Comics (Nintendo Power)
Metroid Prime Comic
Metroid Prime: Episode of Aether