The plasma beam is a heat-based mode for Samus's arm cannon. It's exact function varies, but it is always one of the most powerful beam weapons in Samus's arsenal.
In the Prime series, the plasma beam fires lances of flame which set enemies on fire, and melts certain barriers. If an enemy survives being hit by the powerful beam, it takes additional damage for a time while it burns.
Outside the Prime series, the plasma beam pierces through enemies rather than set them ablaze, causing exceptional damage. Certain high-powered enemies can only be damaged at all using the plasma beam, though other beams may be used in conjunction. The plasma beam tends to be green-colored in these games.
In the Super Metroid series, the Plasma Beam can be used in a Special Attack:
1. Equip the Charge Beam and ONE other beam ability e.g. ice or wide (NOT two)
2. Select the power bombs but do not enter morph ball mode.
3. Charge your beam up and you should use one of the following secret attacks:
Ice- A shield will form around you damaging enemies you touch
Wave-Energy Balls circle you then Fire away from you in various directions
Plasma-Same as wave but with Plasma effect
Spazer- Large beams fire away from you like shrapnel
4. This will use up one of your Power Bombs (It may not work if you've just finished charging, give it another second)