Traps are relatively inexpensive structures (although the Giant Bomb costs 20,000 gold) that remain hidden until they are activated by an attacking troop moving into their trigger radius.
Once activated, traps have a one-time effect and are then consumed. After being activated, traps must be repurchased in order to be a part of the next defensive Layout. Untriggered traps can be sold for 20% of the original price.
Although traps can greatly improve the effectiveness of a village's defense if properly placed, once triggered they are destroyed regardless if the attack was successful or not. If for example your village successfully repels an attack while you are sleeping, but several of your traps are triggered, any subsequent attacks will not have to deal with those traps until you replace them.
Traps cannot be upgraded. The primary consequence of this is that explosive traps (especially the standard Bomb) grow steadily weaker from a relative standpoint as troop levels increase. Spring Traps are the exception as they can kill higher level troops as easily as lower level ones.
Although commonly thought of as a trap, the Hidden Tesla is not listed here as it actually a hidden Defensive Building and does not need to be repurchased for re-use.
Even though traps are classified as "Defensive Buildings", they are not buildings and do not need to be triggered/destroyed for your enemy to reach 100% destruction (nor do they count toward it).
"Nothing says 'STAY OUT' quite like a good old-fashioned hidden bomb."
Explodes when an enemy comes into proximity. It works well near outside of village so larger areas of troops are weakened before breaking through.
Does splash damage.
By itself, a single Bomb will not do a great deal of damage to even the weakest of troops (although every bit helps). However, several Bombs in a cluster can significantly damage or even destroy an entire group of Archers or Goblins, for instance.
The bomb takes a little time before it can explode.
Helpful to put near or around town hall so you can get rid of large numbers of troops closing in.
A sound strategy for Spring Traps is to place them in an intentionally-designed wall gap. As troops will generally tend to go through the gap rather than fight their way through the wall, they will be directed right over the trap and be bounced to their death. Remember, however, that Spring Traps are temporary structures and until replaced will leave an undefended gap in the wall after an attacker has triggered it.
An improvement to this strategy is to place walls inside your base so that the enemy troops have to walk over the Spring Trap to attack nearby buildings.
"When you're looking for a Big Boom, you need the Giant Bomb."
Explodes when an enemy comes into proximity. Essentially a much more powerful (and expensive) version of the Bomb. This creates splash damage to all enemies within a 3.5 radius.
Because of its high price, in practice it is only cost-effective to use the Giant Bomb if it is going to save you from losing more than 20,000 Gold. Even in this situation it is commonly thought that it is not worth the expense, as it possible to be triggered by (and kill) a single inexpensive unit like an Archer or Barbarian.
If you love using Giant Bomb and you want to get good amount of troops with it, then place it where you commonly see a large amount of troops deployed when you are being attacked.
A Giant Bomb takes up the same amount of space (2x2) as a Hidden Tesla. If you have multiple places where your Hidden Teslas could spawn, placing Giant Bombs in the others can be effective, as your enemy may decide that the area is safe once he does not trigger a Hidden Tesla and spawn additional troops.
"Latest innovation in the field of flying pest control. This trap can blast multiple air units in a small area."
Explodes when a flying unit comes into proximity. Essentially an airborne version of the normal Bomb, though the trigger radius and blast radius of the Air Bomb is much greater.
Extremely effective against large groups of Minions, and does a significant amount of damage to Balloons (especially level 3 and below). Much less effective against higher health targets such as Dragons.