Hazard refers to anything that can kill or stun a Pikmin outside of enemies.
Fire is one of the hazards in both Pikmin games. They usually come from a fire geyser, that can only be destroyed in Pikmin 2. It can also come from enemies like Fiery Bulblax, Fiery Blowhog, Fiery Dweevil and enemies like that. In Pikmin, they die from fire very quickly, while in Pikmin 2, it takes a while for them to die. Fire does not spread between Pikmin. Olimar, Louie and the President can block fire when they get the Forged Courage item in Pikmin 2. Red Pikmin and Bulbmin are immune from fire. Some gates are covered by fire geysers, meaning that only red Pikmin, and Bulbmin can break them.
Electric is possibly the most dangerous hazard in Pikmin 2, it did not appear in Pikmin, When Pikmin touch it, their dead, you can't save them. It usually comes from broken electrical circuits form the ground, enemies like Anode Dweevil and Anode Beetle give off electric. Getting the Dream Material will make the captains withstand electricity. There wasn't electricity in Pikmin because Yellow Pikmin can use Bomb Rocks. Yellow Pikmin and Bulbmin don't get effected by electricity.
In Pikmin 3 forward, Electricity is no longer an instant kill and only stuns them for a short while, making it the least deadly hazard.
Water includes small areas of water and water shot at Pikmin by enemies. If Pikmin even touches water, they will start to drawn, they can be saved by using your whistle over and over. Blue Pikmin have gills and can save other drowning Pikmin when idle, Bulbmin also have gills, but won't save other Pikmin. Pikmin won't carry an item into water expect for blue Pikmin because they know that water will kill them. Sometimes you can clog the water by attacking a covered hole in water. Some enemies will shot water like Caustic Dweevil and Watery Blowhog, Pikmin will run around with water stuck on their heads. Call them back to save them. In Pikmin, Goolix was the only enemy that did this. There are gates in water, so only blue and Bulbmin can break them. Water does not hurt captains, however if an enemy shots water at you, you will by stunned, and won't take damage. Blue Pikmin and Bulbmin are immune to both.
Poison only in Pikmin 2 usually refer to small pipes sticking out of the floor that suffocates Pikmin to death. Whistling them saves them. Enemies like Doodle Bug spits out poison. Only White Pikmin and Bulbmin can break these, there are also gates with poison around them. Captains won't get hurt by this. It wasn't in Pikmin because White Pikmin and Bulbmin were not discovered yet. In Pikmin 3, it is only featured in Challenge Mode since White Pikmin are not available elsewhere
Crushing mainly refers to falling rocks and crystals. This instantly flattens any Pikmin underneath the rock except for Rock Pikmin who get stunned for a little bit. Captains also get significantly damaged and stunned for a little bit. In the original, only enemies like Wollywogs could crush Pikmin but, in Pikmin 2, falling rock hazards were introduced though in multiplayer, since they were a bonus, they only sent Pikmin back to the onion.
In Pikmin 3, they clarified it to rocks and crystals specifically with Arctic Cannon Larvae firing snowballs that do not kill.
Explosions have been in since the original game though any Pikmin deaths in the original were self inflicted since they were only achieved from Bomb Rocks. In the second game, some enemies incorporated bomb rocks into their attacks and some enemies could fire explosions. In Pikmin 3, those enemies did not reappear.