Spectre involves finding some code in the kernel that has access to the data, and getting it to access it speculatively before the conditional that tells it not

Spectre involves finding some code in the kernel that has access to the data, and getting it to access it speculatively before the conditional that tells it not to is evaluated. Meltdown involves crafting some code yourself that speculatively accesses data it does not have access to before the CPU rejects the access due to permissions. In other words, Meltdown involves your code accessing the data, but Spectre involves getting the kernel to do it for you. Spectre is a neat clever side-channel attack that is hard to protect against, while Meltdown is a blundering error in the way the CPU works.