What really was the Cold War and why did it end so suddenly?

Still trying to wrap my head around this stuff but what really was the Cold War? Like, was it just a constant fear of nuclear war between the US and the Soviets? I get that there was a whole ideological aspect with capitalism vs communism and the whole Red Scare, but I don't quite understand what sparked it all and what made it come to an end in the early 90s. I recently read somewhere that there was a significant thaw in relations around 1985-86, but I'm not really clear on the context of that or what happened afterwards. Anyone else have a good grasp of the whole thing and can explain it to me in simple terms?