Biological units are StarCraft and StarCraft II units that consist primarily of non-mechanical components.
All zerg units and structures are biological and can self-heal.
StarCraft
Biological units are vulnerable to a wide array of effects, such as a science vessel's Irradiate ability and the queen's Spawn Broodling ability. However, they are immune to effects such as the ghost's Lockdown ability.
They include all zerg units, terran SCVs (also counted as mechanical), marines, firebats, ghosts and medics, and protoss zealots, high templar, and dark templar.
StarCraft II
The science vessel's Irradiate can still damage anything that is biological and the queen, medivac dropship, and medic can heal anything biological.