Originally the plane of Argentum, created by Karn the Silver Golem, Mirrodin went to hell when he left. The guardian he set in charge, Memnarch, grew power-mad. The world of Mirrodin is made primarily of artifacts, and is orbited by four moons (or suns, as some cards reference them.) These moons represent Red, Black, White, and Blue mana. Green is absent, until one of the characters becomes the conduit for it's return. The five main parts of the world are the goblin-infested Oxidda Chain of mountains, the elf-home Tangle, the zombie-swarmed Mephidross Swamp, the Vedalken's Home in the Quicksilver Sea, and the leonin tribes of the Razorgrass Fiel d.
Mirrodin - The first set in the Mirrodin block, the plane of Argentum was renamed to Mirrodin, after the Mirari. Memnarch was left in charge and is responsible for the name change, as well as most of the events in the block. Mirrodin brings a lot of changes to the game, including a whole new card design, making artifacts almost white in color, compared to the brown they used to be. It's a shift away from old dusty tomes and fragile clockwork, and towards polished and deadly machines. Mirrodin introduced a cycle of lands that were also artifacts, which played in with the Affinity mechanic. Affinity reduced the cost of a spell per how many artifacts are under your control. Imprint was a mechanic where you exile a card when another comes into play, giving that card either special effects or the ability to function. Spells with Entwine had two effects you could choose from, and a cost to pay if you wished to play them both. Mirrodin is also the origin of the Equipment subtype of artifacts. Mirrodin's set symbol is the Sword of Kaldra, an avatar created when three artifacts are brought together.
Darksteel - This set has the honor of having the first card banned since Masques, the Skullclamp, a powerful card-drawing engine. Darksteel follows the plot of Glissa Sunseeker as she hunts down the machines that killed her family, visiting each area of Mirrodin in turn. Darksteel introduced three new mechanics; Indestructible, Charge counters, and Moduar. Modular creatures were 0/0 and came into play with +1/+1 counters on them, and when they die, the counters can be moved to another creature. Charge counters were just regular counters put on an artifact, which would be spent for some effects or made other effects more powerful. Indestructible means t he permanent can't be destroyed and for creatures lethal damage doesn't kill it. The only way to kill indestructible creatures is to reduce their toughness to 0, via -1/-1 counters. Darksteel's symbol is the Shield of Kaldra.
Fifth Dawn - Fifth Dawn further modifies the artifact card design, with the original change making them look too much like White cards. They're now closer to silver. Fifth Dawn's plot centers around the rebirth of the Green Sun, long absent from the plane. It's primary new mechanic, sunburst, reflects that. Sunburst cards get +1/+1 counters or charge counters, depending on if it's a creature, for each color of mana used to play it. Fifth Dawn also keyworded Scry, which is the "look at the top X cards of your library and put any number of them on the bottom of your library in any order, and the rest on top in any order" mechanic. This set's symbol is the Helm of Kaldra.