Reapers are faster, stronger, and more intelligent than creepers, and they can explode multiple times, making them very dangerous. These terrible monstrosities have ten hearts, making them hard to kill. They can spawn in only one biome, Hell, the Nether. The strength of a reaper's explosion depends on how long the reaper charges up for. If it waits 10 seconds to load, the reaper's blast will be powerful enough to destroy diamond blocks. Reapers also can jump ten blocks high, and even climb any surface. So whatever you do, do not aggravate a reaper if you value your life.
If you are looking to kill one, your best shot would be to use rapid fire with arrows. Though Diamond Swords can do a significant amount of damage, the Reaper's explosion would kill you. Instead, make sure you have plenty of arrows and attack from high above the ground.
In DoM canon, Reapers are part of the evil Nether army trying to take over the world. Bleek almost killed Alexander when he mistook him for a Reaper due to his red shirt.