A large mechanic of the Metroid series is the capabilities of Samus Aran's power suit. Every Metroid game has item pickups that upgrade the suit, allowing Samus access to new areas, in addition to giving her an easier time against tougher enemies. Her suit was designed specifically for her, and is molded after Chozo battle armor. It has ability to change forms and colors by absorbing Chozo artifacts. The Chozo were chummy with a few other races, namely the Luminoth. Samus's suit can interface with suit data from these other races as well.
Though the Power Suit is Samus's default suit, and the official name of her Chozo-designed suit, she is rarely seen in it; Samus's more familiar form is in her Varia Suit. The Power Suit is a modified version of Chozo battle armor made specifically for Samus Aran's human body. It is what allows her to access various upgrades like the Morph Ball and other suits. The suit's visor apparently allows Samus to interface with almost any kind of computer system, lock-on to moving targets, and even recognize distinct species and then download detailed profiles on them.
The Varia Suit protects Samus Aran from varying temperatures. This suit is also her most well-recognized form. The most noticeable physical differences from the Power Suit are the big shoulders and huge breastplate.
The Gravity Suit allows Samus to move normally in any environment by simulating the gravitational conditions she is accustomed too. This difference is most applicable underwater, e.g. it allows Samus to make higher underwater jumps. The most noticeable physical differences from the Varia Suit are the purple arms and legs, and the blue visor.
Samus gets the Phazon Suit in Metroid Prime when she becomes saturated in Phazon. With the Phazon suit she becomes immune to blue Phazon, but is still weak to orange Phazon. It's more of a mutation in the biomechanics in her suit than it is a new suit itself. She only has the suit for the second half of the first Metroid Prime game, and then it's gone forever.
In Metroid Prime 2, the Dark Suit reduces damage taken from the Dark Aether atmosphere. The atmosphere on the dark side of Aether is usually harmful to Samus's health, but the Dark Suit allows her to venture more boldly into the abrasive territory. The upgrade also increases the suit's ability to absorb damage. The dark suit's Gravity Boost attachment clips onto Samus's back and basically emulates the effect of the Gravity Suit.
In Metroid Prime 2, the Light Suit gives Samus full immunity to the Dark Aether atmosphere. Again, the upgrade makes her suit more resistant to damage. Additionally, the Light Suit gives Samus the ability to teleport between four key points in Light Aether (the central Luminoth temple, and each of their three satellite temples).
In Metroid Fusion, the Fusion Suit is created as a result of Samus being attacked by an X Parasite, and saved using Metroid DNA. The suit grants her Metroid-like aspects, like more acrobatic prowess, and most importantly the ability to absorb X Parasites by making contact with them. The absorption both destroys the X Parasite and allows Samus to pick up ammunition and power. Unfortunately, she also gains the Metroids' weakness to ice.
First seen in Metroid: Zero Mission, the Zero Suit is worn by a more vulnerable Samus who must rely more on speed and stealth than power weapons. This suit of light armor is blue and shown to be worn by Samus under her Power Suit in some games, and has the same force shield defensive barrier. The Zero Suit's light armor does not protect Samus's head. In addition, the Suit's right gauntlet contains a Thermal Positioner similar to that of the Power Suit. Although the Zero Suit apparently retains all of Samus's collected upgrades, they are all completely unusable, with the exception of the Energy Tanks. In Metroid Prime 2: Echoes, Metroid Prime Hunters,and Metroid Prime 3: Corruption, the suit is seen in the ending sequences.
Samus carries an auxiliary pistol (referred to on the official Smash Bros. site as the Paralyzer) for use in the case of total failure of the Power Suit's functions. The shots from the pistol have the same effect as a morph ball bomb when used against vulnerable structures, and do not generally have any effect on well protected objects. The pistol charges up while not firing, and it can momentarily stun Space Pirates when firing fully-charged shots. Samus is a playable character in Super Smash Bros. Brawl while wearing her Zero Suit. Samus will shed her Power Suit and reveal her Zero Suit after performing her Final Smash attack. In her Zero Suit form, she will be weaker, but faster, than normal Samus.
"Justin Bailey" Suit
In the original NESMetroid, finishing the game in a very short time (or entering a code gained after having done so) removed Samus's suit in the subsequent play-through. This had no real effect on the game.
The JUSTIN BAILEY code was one of the most well-known codes that caused this effect (though, when you enter the code, you start off with this suit and lots of other upgrades and the Varia Suit upgrade could be picked up as normal). The JUSTIN BAILEY code is rumored to be intentional on the part of the designers; however, this is false.
One myth (the most popular) states that "bailey" is British slang for "swimsuit", and that the code is meant to say "just in bailey". This is false; "bailey" is not slang for swimsuit in any language. Another states that Justin Bailey is the name of a person who had some sort of involvement with the development of Metroid, or was a friend of the designers, or something along those lines. This is also false.
In reality, it's entirely coincidental. The password system that Metroid uses has resulted in a number of codes that look meaningful, but in reality are simply coincidence; JUSTIN BAILEY happened to be the first one that was discovered and publicized. The only intentional password put into the game was "NARPAS SWORD0 000000 000000", or "NAR (North American Release) Password 000000000000".