Brinstar is, in the Original Metroid and Zero mission the starting point of the game. It also reappears in Super Metroid, but not as the starting point, but the second main area you explore just after Crateria.
Brinstar in Zero Mission/Metroid (NES) differs from that of Super Metroid, in which the original and Zero Mission have Brinstar portrayed as this rocky, barren cavernous area mostly devoid of plant life. Super Metroid however portrays Brinstar as a lush Jungle paradise filled with all sorts of exotic plants.
Its entirely possible that these sections existed in their respective games, just not explored. Further supporting this is the fact that the cavernous rocky section did appear in Super Metroid, although it's only a few rooms and one of these rooms happens to be the starting point of the original Metroid, where you first acquire the Morph Ball.
In the original Metroid game for NES, Brinstar is the first area you explore and the very left from your starting point is where bombs are picked up. This room also retains it's exact layout even to where the Morph Ball is picked up in Super Metroid.
A noticeable room located in Brinstar is the "Statue room" which leads to the entrance to the games' final area, Tourian.
The boss of Brinstar is none other than Kraid, although much shrunken down compared to his future encounters. He is located in his own little subsection of Brinstar called Kraid's lair. Defeating him destroys one the statues in the statue room and awards you Missiles.
Kraid's lair differs itself from actual Brinstar with a change in music, and the fact that Kraid's lair has much more plant life.
 Super Metroid
Brinstar once again reappears in the sequel, Super Metroid. This time around Brinstar is not the starting point of the game, however, the first room you explore in Brinstar which is taken from an elevator from the original Tourain happens to be the exact same room as the starting point in the original Metroid, with the Morph ball upgrade in the same spot as before. This upgrade is being guarded by some kind of security camera which alerts Space Pirate forces in Crateria.
Brinstar is spilt into 4 main sections; green jungle Brinstar, Red Brinstar, Kraids lair and Blue rock Brinstar.
The first part you explore is blue rock Brinstar, as mentioned earlier and is where you obtain Morph Ball.
Unlike the original Metroid, only a few rooms remain and the entrance to the long shaft, right to the room where an energy tank has been sealed in the ceiling has been blocked off.
"Green Brinstar" is the next section you explore with this being the most maze like. The music is energetic and jumpy and the area is filled with exotic jungle plant life and insectoid based enemies.
The Charge Beam is found in brinstar in the "pink rock" room that is accessed by going right.
The very top right to this large pink rock room is a section where some Kihunters can be found and the hatch at the end of this room leads to Spore Spawn the first boss. It guards the Super Missiles, which are used to access Red Brinstar.
 Red Brinstar
Red Brinstar can only be accessed through a Super Missile hatch.
Red Brinstar is not as maze like as green Brinstar although the first room is a huge shaft and going upwards requires the Ice Beam, unless you are planning on sequence breaking, which requires a tricky sequence of Wall Jumps and a shot to break part of the ceiling.
Going downwards, however is the key to progressing the stage and features a point of no return until you have the Ice Beam as the Rippers cannot be stood on unless frozen.
Entering a hatch to the right and the second room to the right leads you to a room with water and some plants that try to eat you. This room has a secret to it where if you shoot a block furthest to the left on the ceiling, gains access to a room where you find the Spazer Beam.
As you keep progressing right you will come to a glass tunnel that is popularly known as "The Maridan tube." This tunnel can be shattered with a Power Bomb, and is one of the 2 main entrances to Maridia.
After this room is the elevator room to Norfair however this room contains a fairly well hidden secret in which the first 3 blocks to the right of the room can be shot with Super Missiles which leads you to Kraid's Lair.
Unlike the Original Metroid, Kraids lair now is metallic and industrialized, as opposed to being mainly composed of organic rocks and plant life.
This area is the smallest subsection of Brinstar and contains some of the most Space Pirate activity, including Kihunters and Green Space Pirates. Kraid himself is guarded by Mini Kraid, similar to the original Metroid.
The room directly before Kraid contains a rather mysterious dead body. It is unknown who or what he is or how he died. Although its possible the eye door killed him before he could get to Kraid. presumably his mission.
Kraid's battle takes place in a huge room with some stones that are the same rock found in "Old Brinstar". As the battle starts, Kraid rises from the ground to do battle. he is rather simple and only requires missles or Super missles to be shot in his mouth when he roars.
When he dies he sinks back into the ground and the Varia suit is now unlocked.
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