From BattleTechWiki:
Plasma weapons use the power of the fusion engine or power amplifiers to power an electromagnetic accelerator that fires a stream, pulse or toroid of plasma (i.e. very hot, energetic, and excited matter). The plasma is created by using a lasing process to flash cartridges of plastic foam into white-hot projectiles that cause thermal as well as physical damage.
The primary damage mechanism of these is thermal transfer. The plasma weapons may also have a significant kinetic energy component, that is to say the ionized material is projected with sufficient momentum to cause some secondary impact damage in addition to causing high thermal damage.