Philip K. Dick is, without a doubt, one of the greatest science fiction writers in the history of the United States. You can't overestimate his legacy and his influence on the genre and the modern-day culture. As for Second Variety, it's a short story by the master that first saw the light of day back in 1953. The Soviets and the Americans started a nuclear war that turned the once-beautiful planet Earth into a never-ending wasteland.
At the same time, the war is far from being over, and the fight goes on even when there's pretty much nothing to salvage. Humanity is weak and vulnerable, but they are still at each other's throats. The Western allies managed to create fully automatic, self-replicating robots that are supposed to turn the tide on the battlefield. They call them "claws". Second Variety is one of those classic Dick short stories in which the radiation from the nuclear missiles makes our planet's surface deadly and uninhabitable for the humans.
Now, despite the fact that this wasn't a full-length novel or anything like that, the international critics praised Dick for delivering one of his most compelling, engaging and thrilling stories and depicting a complex, diverse and realistic post-apocalyptic world. Back in the 50s, there weren't many stories, movies or comic books in this particular genre, and Dick was one of the first ones to find appeal in it.
By the way, in 1995, the movie makers turned Dick's tale into Screamers, a film that was "smashed" by the critics but received the status of a cult. Today, more than 5 decades after the release, Second Variety is still a pretty entertaining read, not to mention it's the "Godfather" of all the modern-day apocalyptic stories that have to do with rebellious robots, including Battlestar Galactica, The Terminator, et cetera, et cetera.