Lava
Lava is, quite simply, molten rock. It can be found as a hazard in many parts of Mushroom World.
When Mario touches lava, it can scorch him terribly, causing him to run around, almost uncontrollably, in pain. In some instances, it may result in him losing some health (as seen in Super Mario 64 and Paper Mario), or instant death (as seen in Super Mario Sunshine and most 2D Mario games, such as Super Mario Bros. and Super Mario World).
In Super Mario Sunshine, in addition to the lava found in Corona Mountain, the Goop painted by Bowser Jr. (a.k.a. Shadow Mario) with his Magic Paintbrush would occasionally contain traces of lava. This lava did not result in immediate death, but Mario would leap into the air uncontrollably, covering little horizontal distance. Therefore, if there is a lot of the lava goop when Mario touches it, death may still be inevitable.
In the Mario Kart series, lava appears in many race courses. Falling into lava delays the racers, as they must wait for a Lakitu to pull them out.
Despite lava appearing in almost every Mario game (or almost every video game for that matter), the only time lava is seen in Paper Mario: The Thousand-Year Door is during two Bowser events; the first around Bowser's Castle, and the second in a stage reminiscent of Super Mario Bros.. Mario himself never encounters any lava at all. In Super Paper Mario, there is no lava at all, except for the lava around Bowser's Castle in the introduction screen.
Lava has also been shown to start Mario's overalls on fire. Why it does not burn a hole through is unknown.
Donkey Kong Series
Lava is, like on Earth, a naturally occurring and dangerous substance in the Donkey Kong Universe.
Lava's first appearance in a Donkey Kong game was in Donkey Kong Country 2: Diddy's Kong Quest, where it was an obstacle found primarily in the area Crocodile Cauldron, a large volcanically active area.
In Donkey Kong Country 2's semi-sequel Donkey Kong Land 2, lava is slightly less common due to Crocodile Cauldron being fused together with the large and swampy Krem Quay to create Krem Cauldron.
In Donkey Kong Country 3: Dixie Kong's Double Trouble! lava makes a small appearance in the hidden world, Krematoa. Here, a small pond of lava can be seen and oddly enough, swam through after Krematoa's ancient volcano is re-activated.
This same pond of lava can be seen in Donkey Kong Land III, the pseudo Game Boy sequel of Donkey Kong Country 3.
Lava also appears in Donkey Kong 64, once again as an uncommon, yet dangerous obstacle.
