Motion Sensor:
Any motion from an enemy sets it off.  If places near a turret or base defenses will allow them to target the enemy and fire.

Pulse Sensor:
Sends a pulse beam out and receives signals back.  Based on that reply any turrets can track and fire upon you.

Sensor Jammer:
Blocks all pulse sensor activity, but has no effect on a motion sensor.

Guardian:
Can only be plaved near the flag. If the enemy comes near the location the guardian then alearts all team members and explodes when the enemy gets closer.