As this land enters, you may pay 2 life. If you don't, it enters tapped.
Steam Vents is a premium dual land and a near-auto-include in any Izzet (UR) deck that runs both colors. The reason it's so good is flexibility: it counts as both an Island and a Mountain, so it fetches with any land tutor keyed to basic types — Fabled Passage, Prismatic Vista, Misty Rainforest, Scalding Tarn, etc. That synergy with fetchlands is the whole point, and it's why shocklands sit above almost every other untapped dual.
The "pay 2 life or enter tapped" choice is real upside in Commander, where life totals are 40 and you often have time. You get untapped mana when you need to curve out, or you keep your life when the board is calm.
Play it in any two-color (or more) deck touching blue and red — spellslinger, control, artifacts, storm, whatever. The only reason to skip it: budget builds, mono-color decks, or extremely slow casual pods where the life loss isn't worth it and you'd rather just play a tapland.
This land has two basic land types. It's not basic, so cards such as Tend the Sprigs can't find it, but it does have the appropriate land types for effects such as that of Kishla Village (from the Tarkir: Dragonstorm release).
If an effect puts this land onto the battlefield tapped, you may pay 2 life, but it still enters tapped.
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