Mirror World
Mirror World is a universe that exists parallel to Kirby's world, and is the setting of Kirby & The Amazing Mirror. Its entrance is located high above Dream Land; through a shining, golden, mirror-like portal. The Mirror World itself is a tiny round planet in space, with a large variety of environments.
Unlike Dream Land, every section of the Mirror World is united by an area located at the very heart of the Mirror World itself. This area is known as the Rainbow Route. Rather than Star Doors, the Mirror World has Mirror Doors, which are basically mirror-like portals. There is also a much larger ratio of the Mirror Doors in the Mirror World to the Star Doors on Pop Star. This makes it a complex maze with many entrances and exits to the locations.
Strangely, the Mirror World also has parallels of many of the landmarks and inhabitants of Pop Star. For example, in Dream Land (more specifically Ice Island), there is a large, icy tower that looks like Mirror World's Peppermint Palace. The same can be said about Moonlight Mansion and Castle Lololo and other such buildings. As for villainous folk, King Golem and Whispy Woods are shaped and behave almost exactly the same. Other correspondents like Wiz and Paint Roller are less obvious. The most obvious one is Meta Knight and Dark Meta Knight.
Areas
Rainbow Route
The first level Kirby enters, Rainbow Route, has no boss of its own. It is divided into sections that include forests, caves, conifers, and mountains. Each of these sections will lead Kirby to further levels- most particularly one section of Rainbow Route that serves as the game's formal, main hub (which looks like platforms in a cloudy sky).
Moonlight Mansion
A mansion located in a part of the world which seems to be in a perpetual state of night. Surrounded by a graveyard, Moonlight Mansion appears eerie on the outside, but holds many intriguing secrets on the inside.
Boss: King Golem
Cabbage Cavern
A large, purple, underground cavern. Inside, there are many confusing pathways, as well as underground water reservoirs with strong currents.
Boss: Moley
Mustard Mountain
A seemingly harmless tall peak. But when Kirby begins to scale it however, he finds out soon enough that it's actually an active volcano!
Boss: Kracko
Carrot Castle
A castle hidden away in a large forest. Inside exist many long and winding corridors, and surprisingly strong gusts of wind along the roof.
Boss: Mega Titan
Olive Ocean
A dark blue ocean, with many islands springing about the surface, and a large coral reef underneath.
Boss: Gobbler
Peppermint Palace
A large icy region divided into two parts. One area is an ice-covered mountain range. The other is a palace made entirely out of ice and snow located in a secluded forest.
Boss: Wiz
Radish Ruins
Ruins of an ancient culture, hidden away from the test of time. Kirby will pass through not only the ruins, but also a mountain range and field at sunset, before facing off with Meta Knight's mirror counterpart for the first time.
Boss: Dark Meta Knight (called "???" during the fight and disguised as Meta Knight)
Candy Constellation
Candy Constellation is located high above the skies of Mirror World, in space. Hidden well, the only way to reach it is to find a special warp star in one of the other seven levels.
Boss: Master Hand and Crazy Hand (fought simultaneously)
Dimension Mirror
If all eight Mirror Shards are found, the mirror will become whole again, allowing Kirby to face the evil ones trying to take over the Mirror World.
Boss: Dark Meta Knight and Dark Mind