Scythe is a tool of both agriculture and combat. It can be used to harvest crops, in which case it only breaks fully grown crops. In order to increase its efficacy in farming, enchant it with Sweeping Edge and harvest multiple crops at once.
Enchanting it with Fortune will increase yields in harvesting.
There is also a special enchantment named Guillotine if you are into harvesting heads instead of crops. This enchantment adds a charged right-click attack to the Scythe. If you deal a killing blow with the right-click, you can decapitate your foe and use their head for decoration.
With Guillotine, you can not only harvest player heads but also the heads of some other mobs. You cannot harvest heads that can be normally gathered like zombies, creepers and wither skeletons.
In addition to these enchantments, Scythe can make use of Unbreaking, Mending, Sharpness, Smite, Bane of Arthropods and Looting as well.
Devour is a special combat enchantment for the Scythe. It lets you drop little hearts out of the enemy you strike, which you can pick up to replensish health! There is a chance to drop a heart at each hit depending on the Devour level, and a certain amount of heart drops at the killing blow, that again vary depending on the enchantment level.