Anti-Ballistic Missiles are launched automatically whenever an approaching Interplanetary Missile is detected. Otherwise they do not take part in any attacks, thus they can not be sent on missions or destroyed by an attacking fleet.
You can store 10 Anti-Ballistic Missiles for every Missile Silo Level on your planet (twice the amount of Interplanetary Missile).
You can deconstruct both Interplanetary Missiles and Anti-ballistic Missiles.
Since OGame 0.78a, IPMs can be used to destroy defenses on a moon and ABMs from the planet will defend the moon in those cases.
Anti Ballistic Missiles (ABM) are your only line of defense when attacked by Interplanetary Missiles (IPM). When a launch of IPMs is detected, these missiles automatically arm, process a launch code in their flight computers, target the inbound IPM, and launch to intercept. During the flight, the target IPM is constantly tracked and course corrections are applied until the ABM reaches the target and destroys the attacking IPM. Each ABM destroys one incoming IPM.