Tallon IV (IV as in the Roman Numeral 4) has 7 areas for you to visit, but they aren't unlocked easily. In order, here's what you'll see in these areas.
Space Pirate Frigate, Orpheon
Orbiting the planet Tallon IV, this is where Samus begins the game. The frigate's name is Orpheon. The Space Pirates used this as a Research Facility for studying organisms from Tallon IV, i.e., natives of the planet. When Samus arrives there, the whole place is ridden with dead and injured Space Pirates and scattered metal.
Later this crashes into Tallon IV and Samus then explores in it after acquiring the Gravity Suit, since most of the rooms are below sea level.
A rainforest-like area, this is where Samus initially makes planetfall and is where Samus's Gunship stays for the duration of the game. Tallon Overworld also encompasses the sunken Orpheon.
Despite being the furthest area to the southeast on the map, this is considered to be the central area of the game, since you'll be visiting this area frequently. One reason for this is that it includes the Artifact Temple, where you'll be collecting 12 artifacts scattered across Tallon IV, and the above-mentioned Orpheon ship.
Far northeast on the map, this ancient desert-like structure was once a center of the thriving Chozo civilization on Tallon IV that was brought to an end by the meteor that hit many years ago. It houses an elaborate system of rooms and also has some straightforward but fun puzzles to do.
This area is central in every sense of the word. For instance, it is by the center of the map, slightly west, and extends to the far north. Furthermore, it is visited as frequently as the Tallon Overworld. Acting as a subway system for the game, its magma-filled tunnels connect all the areas together with the exception of the Impact Crater. The Space Pirates use the caverns as a source of geothermal power.
The one unique feature about this place is that it doesn't have a boss. As said, it's a connection between all of the worlds except the Impact Crater, but you won't see a single boss here. Breathe a sigh of relief for this, as that most of the rooms in the caverns are almost 60% filled with hot magma or fire.
This cold, mountainous location is the westernmost part of the map, ironically accessible only by using the elevators in Magmoor Caverns. Phendrana consists of three main parts: an ancient Chozo ruin, a Space Pirate research lab (since the subzero temperatures make specimen containment easier), and the ice caves and valleys at the edge aptly called Phendrana's Edge. It is home to many creatures of electrical origins (Pulse Bombu, Scatter Bombu) and ice based creatures (Sheegoth, Ice Shriekbats).
The Phazon Mines is the southernmost region on the map and the center of the Space Pirates' Tallon IV operations. The scenery here alternates between the Space Pirates' mining and research facilities to dark caves illuminated only by the glowing Phazon. Also home to the different beam troopers, Elite Pirates, the Phazon Elite pirate, and the super-powerful Omega Pirate as well.
Not only does this place house powerful enemies; the whole area is HUGE! The map, as an example of this, literally divides the areas into 3 different levels: Level 1 is the entrance to the mines; Level 2 is the Space Pirates' main mining and research area; and Level 3 is the Phazon and Fungi area, which has almost, if not more, the amount of Phazon and gigantic mushrooms in the rooms as Magmoor Caverns has magma.
This is the final area in the game. This location is completely corrupted by Phazon radiation. Samus finds Fission Metroids, special Metroids that can split into two Metroids, and Phazon strong enough to damage her even through the Phazon Suit. You also encounter Metroid Prime, the final boss in the game.