Technically, Death's Scythe is a sickle; a scythe has a longer handle and a straighter (but still curved) blade, as opposed to the fist-sized handle and the blade making an arc about a half-circle.
This scythe is possibly the most powerful weapon in creation, exceeding an Archangel Blade, The Colt, Angel Blade, and Ruby's knife as each is limited to certain creatures or beings. Although the full power of an Archangel Blade hasn't been revealed.
In Two Minutes to Midnight, Crowley tells the Winchesters that the scythe "kills angels and demons and reapers and, rumour has it, the very thing itself." Whether or not it can actually harm Death is unknown.
The scythe is first seen in the possession of the demon Alastair when he attempts to use it to kill two reapers in order to break one of the 66 Seals to release Lucifer from his cage in Hell. He notes that he "borrowed it from an old friend" who "doesn't really ride a pale horse, but he (Death) has three amigos, and they're just jonesin' for the Apocalypse!" After Alastair is taken into custody by the Angels, it is unclear what became of the scythe (though it would later resurface).
Having discovered Death's location in Chicago, Illinois while on a quest to retrieve the remaining rings of the Four Horsemen of the Apocalypse and return Lucifer to his cage in Hell, Dean Winchester is faced with getting Death's ring away from him against seemingly impossible odds (given Death's power). To help in this matter, Crowley reveals he is in possession of the scythe (having gotten hold of it through a crossroads deal) and gives it to Dean, telling him of its awesome power in the hope that it would give him a fighting chance against Death's power. However, when Dean tries to sneak up on Death in a pizzeria in Chicago, the scythe burns red-hot in his presence, forcing Dean to drop it. It vanishes and reappears on the table where Death is sitting, now returned to its master. Death thanks Dean for returning his scythe and then invites him to join him at the table to discuss the current situation of the Apocalypse being orchestrated by Lucifer. The Scythe hasn't been seen since, and is likely still in Death's possession.