From the Super Mario Wiki
Amps are small, electrifying and spherical metal balls who have appeared as enemies throughout the Mario series. Their name stems from "ampere" (often shortened to "amp"), the standard unit of measurement for the flow of electrical currents.
Super Mario series
Super Mario 64/Super Mario 64 DS
Amps make their first appearance in Super Mario 64 and are featured in several levels throughout Super Mario 64 and its remake, Super Mario 64 DS. Similar to Li'l Sparkies and Hotheads, they move around objects and platforms, and if Mario comes into contact with one, he will be shocked and lose one wedge of health. There is no possible way to defeat Amps in this game. Amps can be found in Bowser in the Dark World, Shifting Sand Land, Vanish Cap Under the Moat, Snowman's Land, Bowser in the Fire Sea, Tick Tock Clock, Rainbow Ride, and Bowser in the Sky. While being electrocuted, Mario will flash and become immobile for about one second.
New Super Mario Bros.
Amps reappear as an uncommon enemy in New Super Mario Bros., in World 3-, World 4- and World 7-. In this game, they appear mostly in groups, and if Mario comes in contact with one, he will be shocked. They can only be defeated with a Mega Mushroom, Koopa Shell or Starman. They will lose their electricity for a moment if they shock Mario or Luigi.
Super Mario Galaxy
Amps can also be found in many areas of Super Mario Galaxy, including the Buoy Base Galaxy, the Battlerock Galaxy, and the Dreadnought Galaxy. They have faces, and their eyes and mouth are both bright red. There also other Amps that are large, face-less orbs, called Big Amps. Similar to Super Mario 64, Amps cannot be defeated. They also appear in the Bubble Blast Galaxy. Here, they'll move in groups of two. The second player (if available) can freeze them in place, however. Also amps appear in Space Junk Galaxy.
New Super Mario Bros. Wii
Amps also appear in New Super Mario Bros. Wii as rare enemies, behaving as they did in New Super Mario Bros. Only six of them appear in the first half of World 3 Tower. They can be defeated by freezing them with icy projectiles from the Ice Flower or Penguin Suit and then Ground Pounding them, touching them with a character using a Star, or by having a frozen enemy thrown at them. The player(s) must dodge them if the player(s) go for the Red Coins.
Super Mario Galaxy 2
Amps reappear in Super Mario Galaxy 2, behaving the same way they did in the predecessor. Amps also have the same appearance as in Super Mario Galaxy. They can now be defeated by using a Rainbow Star. They appear in Sky Station Galaxy, Space Storm Galaxy, and Spin-Dig Galaxy.
New Super Mario Bros. 2
Amps reappear in New Super Mario Bros. 2 for the Nintendo 3DS. They act just like they did in the predecessor and appear in the levels World 4-B and World Flower-. Additionally, Amps can also be seen moving on lines, like Fuzzies.
New Super Mario Bros. U
Amps and its giant variation reappear in New Super Mario Bros. U, appearing only in Snake Block Tower and in Ludwig's Clockwork Castle . They behave like in the predecessor, and there is a new variation of amps interconnected with Bowser's face that conduct electricity with each other.
Mario Party series
They appear in various games in the Mario Party series.
Mario Party 3
Mario Party 4
Mario Party 5
Mario Party Advance
A single Amp appears in Mario Party Advance. There, it appears as a citizen of Town Area and needs help in winning a game in Town Game Room B to end his losing streak. Once the player wins the minigame, Match 'Em, they win the minigame as a reward. Amps also appear in the minigames Amplifried, Cloud Climb and Broom Zoom as enemies.
Mario Party 6
Amps appear in Mario Party 6 as the Zap Orb. Once a player passed over a set Zap Orb, they would lose five coins for each space they moved beyond it for the rest of their turn. They're the only enemies in Circuit Maximus minigame.
Mario Party 7
Amps once again return in Mario Party 7, still in the form of the Zap Orb, which had the same effect as before. However, the player lost three coins for every space instead of five.
Mario Party 8
Mario Party: Island Tour
Mario Party 10
Appearance and traits
Amps originally sported white "cross-popping" veins, joyful red eyes, and rounded metallic lips with a pure red mouth. Since Super Mario 64 DS and Mario Party 7, their appearance has been changed a bit. They now have more jagged yellow eyes and a larger, jagged mouth. Their lips are now white, with a pink tongue and black interior being visible inside the mouth. They are also now black rather than grayish and sport no angry veins.
Names in other languages