Tech Armor - Gives more damage protection than other defense powers. Exclusive to Sentinel. Can manually detonate for an area-of-effect damage blast.
Defense Matrix - Slight damage protection. Can be consumed (purged) to restore the user's shields. Available as a bonus power.
Fortification - Slight damage protection. Can be consumed (purged) to give massive melee damage boost. Available as a bonus power.
Barrier - Slight damage protection. Can be detonated to biotically lift enemies in the air. Good for setting up biotic combos. Available as a bonus power.
Fitness - Passive ability that grants melee and health/shield bonuses. Recommended for most class builds and a available to all classes.
All defense powers (except Fitness) add power encumbrance, therefore powers take longer to recharge when active. However, detonating or purging will remove the power encumbrance effect. To purge or detonate the defense power, simply re-use the power when you are not on a power cooldown. Take note that purging or detonating will also force you on a cooldown.
Also, Tech Armor can stack on with other defense powers for extra damage protection, but power cooldowns take significantly longer to recharge due to both defense powers being active at the same time.