They are fired from small sections of walls, usually marked as faces, and travel quickly in a straight line without any change to their trajectory. They can damage the player should he be in their flight path.
In Doom 64, darts are usually triggered by hidden tripwires or switches with at least one bout of warning (a raised section of the wall exposing the firing panel, said panels being raised from the floor, etc.), though occasionally they will fire without warning. Rarely are darts fired out singularly, as more often than not they will be at least paired. Darts can also damage demons.
In Hexen, there are a number of projectiles that can be shot out from various ports, including darts and dart-like items. The two most common darts include metal-tipped projectiles that merely damage the player and poison-tipped darts that cause successive damage depending on how many darts hit the player at once. Other projectile-based traps shot in a similar fashion include fireballs, large spiked balls, frozen icicles (technically a dart), and Porkalator shots.